Which of the following be about　to + infinitive construction may be used to express an unfulfilled intention?

Which of the following be about　to + infinitive construction may be used to express an unfulfilled intention?
A.The new school year is about to begin.
B.The bus was about to leave.
C.She is about to have a baby in one month.
D.We were about to leave when it began to rain.
正确答案：We were about to leave when it began to rain.