Did you see the traffic accident yesterday?Yes ,I saw it ---

Did you see the traffic accident yesterday?Yes ,I saw it ____ when I ____ past the school gate.
A.happen;walk B.happen;was walking C.happened;walking D.happening;have walked
B.happen;was walking
see sb do sth 所以接原型happen
而意思上是 当我正经过学校门口时看见的 又已经发生了 所以用过去进行时