方形补偿器安装时需预拉伸，对于输送介质温度为250～4000℃时，拉伸量为计算伸长量的（）。
A．50%
B．70%
C．80%
D．100%
参考答案：B
Tom was about to close the window ________ his attention was caught by a bird.
A．when
B．if
C．and
D．till
咽鳃裂
