两对等位基因的杂合子(自由组合)与双隐性类型测交时，后代表现出新类型的个体占总数的[ ]
A．0
B．25%
C．50%
D．75%
答案：C
I wrote _____ my brother last Saturday, but I haven't heard _____ him up to now. [ ]
A. from; to
B. to; of
C. to; to
D. to; from
（）是一种系统需求，描述了与用户相关的操作特征。
A.技术需求
B.性能需求
C.可用性需求
D.可靠性需求
