Luckily a doctor happened to be ( ) when his father collapsed.

Luckily a doctor happened to be ( ) when his father collapsed.
A、at the spot
B、in the place
C、on the place
D、on the spot
【正确答案】：D
【题目解析】：on the spot 在现场，句子含义他父亲发病时，正巧有医生在现场。