天线增益是指什么？
参考答案：
在输入功率相等的条件下，实际天线与理想的辐射单元在空间同一点处所产生的信号的功率密度之比。
Not until they left school _____ how much their teachers loved them and helped them. [ ]
A. they realized
B. did they realize
C. they would realize
D. had they realized
对于每种醇胺，其浓度愈高，腐蚀率（）。
A、愈弱
B、愈强
C、无关
D、不一定
