（）aremountedinpairstoprotecttheboileragainstoverpressure.
A. Safetyvalves
B. Watertubes
C. Steamstopvalves
D. Blowdownvalves
Chainslingsaresuitableforheavyslenderarticles，suchas（）.
A. timberorsteelrails
B. locomotivesorboilers
C. railwaypassengercoaches
D. motorvehicles
Whensteamisbeingraisedonaboiler,thewaterlevelwillnormally（）.
A. dropastheboilerwarmsup
B. riseastheboilerwarmsup
C. remainunchangeduntiltheboilerishotenough
D. riseandfallwiththesteamdemand
Cracksmaybepreventedfromdevelopingatthecornersofweldedplatinginsertsby（）.
A. squaringthecorners
B. roundingthecorners
C. plugweldingthecorners
D. slotweldingthecorners
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