一对表现型正常的夫妇，却生了一个色盲男孩，这对夫妇再生一个色盲男孩的几率是多少？（）
A.1/4
B.3/4
C.1/2
D.1/16
参考答案：A
As a student, you should _____ on your studies rather than playing computer games. [ ]
A. looked
B. focused
C. took
D. put
该二叉树对应的树林包括( )棵树。
A．1
B．2
C．3
D．4
