I was really scared and I didn't know ___________ when I saw a snake that day.
A:how to do
B:when to do
C:where to do
D:what to do
小明骑自行车去上学,他在前一半路程的速度为4m/s,他感觉要迟到了,便用6m/s的速度走完了后一半路程,那么他在整个路程中的平均速度是____ 。