在单缝夫琅和费衍射实验中，屏上第三级明纹对应的缝间的波阵面，可划分半波带的数目为：（）
A.5个
B.6个
C.7个
D.8个
参考答案：C
解析：
He was so poor that he couldn't afford ___ a new coat.
A．to buy his son
B．buy for his son
C．buying for his son
D．his son to buy
测量实际制冷系统压缩机吸气口温度的目的是为了帮助决定运行工况图上的过（）。
