从所给的四个选项中，选择最合适的一个填入问号处，使之呈现一定的规律性。（）
A.A
B.B
C.C
D.D
参考答案：C
He spoke proudly of his part in the game, without mentioning ________ his teammates had done.
A．what
B．which
C．why
D．while
牵引有效建立的条件？
