( )抹灰面积按其长度乘以高度计算。
A．外墙
B．外墙裙
C．内墙
D．有墙裙的内墙
参考答案：B
已知ab=3，a+b=7．
（1）求a2+ab+b2的值；
（2）求（a-b）2的值．
A heavy learning burden leads to unhealthy behaviors such as poor eating habits, which ___ damage the health of students.
A．in its turn
B．in return
C．in turn
D．by turns
