天线是无源器件，天线的增益来源于（）。
A、多振子的累加辐射
B、反射板的聚焦
C、振子的放大作用
D、谐振
参考答案：A, B
Mary three of her classmates invited to the party. [ ]
A. as well; has been
B. and; have
C. as well as；was
D. as good as; were
为了便于对照检杏，测试用例应由输人数据和预期的______两部分组成。
