从结构上看，LD有（），而LED没有
A.有源区
B.谐振腔
C.受激吸收
参考答案：B
取50mL水和50mL酒精混合，混合后的总体积______lOOmL，这一实验现象说明______．
He asked ________ for the computer.
A．did I pay how much
B．I paid how much
C．how much did I pay
D．how much I paid
