In recent decades, scientists have become increasingly aware of the part the observer【C1】______in the scientific process. In the【C2】______place, the observer can work only with his experiences, and these are【C3】______by his senses and the instruments he【C4】______to extend his senses. Ultraviolet light, electromagnetic fields, and atomic particles, 【C5】______. became known to us only as we devised tools【C6】______we could observe their effects. 【C7】______, our picture of the real world is always incomplete. Secondly, the observer is highly selective in choosing his【C8】______. Life is a narrative of ever new and often【C9】______events. At any given moment, an individual is bombarded with sense experiences and can, 【C10】______he desires, expose himself to more. But he is really interested in or concerned with only a few of these. Other experiences are consciously or【C11】______ screened out as irrelevant to the task【C12】______. For example, as we read a book, we are often surrounded by sounds and activities that we【C13】______ , but by turning our attention to them we become conscious of their presence. What a scientist discovers depends, 【C14】______, on what he is looking for—【C15】______the questions he is asking. Thus, academic disciplines differ in their study of human beings in large part【C16】______they ask different questions. Human beings live, 【C17】______, in a house with only a few windows of tinted and curved glass, 【C18】______which we see the outside world. The glass colors and distorts our observations, and its effects can be determined only with【C19】______difficulty. Scientists are increasingly aware of the【C20】______that they work with sense data, not with the world itself. 【C5】

A. to our surprise
B. for example
C. to be frank
D. that is to say