如果σm=0，σn=100Mpa，r=( )。
A．1
B．-1
C．0
D．2
参考答案：B
如图所示，在△ABC中，∠C=90°，∠BAC=60°，AB的垂直平分线DE交AB于D，交BC于E，若CE=3cm，求BE的长．
The reason ______ she gives for not coming to the party is that her mother won't let her come.[ ]
A. what
B. why
C. as
D. /
