When she is put in a situation _______ she has to make a remark on others, she usually changes the  topic.

A．that

B．when

C．which

D．where

羊毛铬花箐R分光光度法测定烟尘中铍时，若室温高于20℃，铍离子与羊毛铬花箐尺生成的络合物的吸光度随温度而变化，因此每批样品测定时，皆需绘制标准曲线。