有一读者问如何做下面一道选择题：
(1)Every morning the little girl would be observed_________the crowded street by her granny.
A. to cross B. to across C. cross D. across
由于would be observed 是被动语态，所以其后应用 to cross。 但如将全句改用主动语态，即应变为：
(2)Every morning the little girl's granny would observe her cross the crowded street.
这样 cross 之前就不应用不定式符号to了。
