-- Why didn’t you stop the ball?

-- ________ I could catch it, a boy from Class Three suddenly appeared and caught it.

A. Until         B. When          C. After          D. Before

—Why didn’t you stop the ball?

—________ I could catch it, a boy from Class Three suddenly appeared and caught it.

　　A. Until

　　B. When

　　C. After

　　D. Before