在纯净的半导体中掺入微量的三价元素就成为（）型半导体，其中（）是多数载流子。
参考答案：P；空穴
He was told that it would be at least three more months _____ he could recover and return to work.
A．when
B．since
C．before
D．that
工农产品交换的“剪刀差”
