气孔减少了铸件的有效截面积，造成局部（），降低了铸件的力学性能。
参考答案：应力集中
I have ______ food at home.
A．lot of
B．a lot of
C．a lot
D．a lots of
.Why did you take a taxi____you could come here by bus?
A．while
B．unless
C．when
D．until
