答案：
解析：解析:
A. There was
B. There is
C. There being
D. There having
A. It was a fine day
B. Being a fine day
C. It being a fine day
D. Because the fine day
A. his luggage was loaded
B. had his luggage been loaded
C. loaded his luggage
D. his luggage was being loaded
A. 为所给方程的解,但不是通解
B. 为所给方程的解,但不-定是通解
C. 为所给方程的通解
D. 不为所给方程的解
解析：试题解析:本题考查的知识点为线性常系数微分方程解的结构.
A. starting
B. to start
C. have started
D. was starting
解析：解析:what were you doing提问的是过去进行时。
A. have shut up
B. had shut up
C. have been shut up
D. had been shut up
