The old lady was standing by the window ____ suddenly she caught sight of a postman riding along.
问题描述：
The old lady was standing by the window ____ suddenly she caught sight of a postman riding along.
A.when
B.while
C.because
D.so
答
A
When：当……时
while:当……时,一般双方都是进行时,强调一种对比
because：因为
so：所以