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The text indicates that the Canadian airport officials seem to have______.
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Which of the following can best replace the title of the passage？
Water
The second most important constituent(构成成份) of the biosphere(生物圈) is liquid water. This can only exist in a very narrow range of temperatures, since water freezes at 0℃ and boils at 100℃. Life as we know it would only be possible on the surface of a planet which had temperatures somewhere within this narrow range.
The earth's supply of water probably remains fairly constant in quantity. The total quantity of water is not known very accurately, but it is about enough to cover the surface of the globe to a depth of about two and three-quarter kilometers. Most of it is in the form. of the salt water of the oceans—about 97 per cent. The rest is fresh, but three-quarters of this is in the form. of ice at the Poles and on mountains, and cannot be used by living systems until melted. Of the remaining fraction, which is somewhat less than one per cent of the whole, there is 10—20 times as much stored underground water as there is actually on the surface. There is also a tiny, but extremely important fractions of the water supply which is present as water vapour in the atmosphere.
Water vapour in the atmosphere is the channel through which the whole water circulation(循环) of the biosphere has to pass. Water evaporated(蒸发) from the surface of the oceans, from lakes and rivers and from moist(潮湿的) earth is added to it. From it the water comes out again as rain or snow, falling on either the sea or the land. There is, as might be expected, a more intensive evaporation per unit area over the sea and oceans than over the land, but there is more rainfall over the land than over the oceans, and the balance is restored by the runoff from the land in the form. of rivers.
Liquid water only exists______.
Like dog's poop on city pavements, rubbish left behind by tourists at natural sites is disgusting.
The funny thing is nobody likes seeing such mess in the places they are visiting but it seems only few really care enough to keep the place tidy. The obvious result is that garbage has become a common sight in forests which attract tourists.
We are certain that in many places park rangers—despite their small numbers, incomparable to the tremendous tourist troops—are already doing the best they can. Sometimes, however, we believe things could still be better.
Take the Thi-Lor-Su waterfall in Tak province for instance. One of the things about Thi-Lor-Su which we would like to talk about today is the commendable method of rubbish control at the waterfall.
There, food and drinks in commercial packaging are prohibited. The ban is enforced by requiring that each and every visitor heading from the camping ground to the waterfall has to pass through a checkpoint where their bags are inspected and potential garbage is seized. The results have been amazing. Despite thousands of visitors, the 1. 5-kilometre trail to Thi-Lor-Su, as well as the waterfall itself, is almost free of garbage.
However, within the same forest, along the 16 kilometers of rough road from the Pha Luead checkpoint to the camping ground, the situation is completely different. Both sides of the road are littered with rubbish, from candy wrappers to Styrofoam food boxes to plastic water bottles.
One contributing factor to the mess is that several tour operators use ordinary pick-up trucks to transport their clients in and out of the forest. The ordinary trucks often get stuck and tourists are forced to walk the rest of the distance—which can be several kilometers. And, as you would expect, there are more inconsiderate tourists than conscientious ones. Along the way, people eat and throw rubbish.
One solution to this problem is to encourage tour operators to use proper vehicles. Either that or put off traveling into the forest until the road is really dry and passable.
How does the writer feel about the fact that people hate to see rubbish but still throw it carelessly？
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