问题补充：
Believe it or not, he was seen upstairs just now.
A. to go
B. to going
C. having gone
D. go
答案：
(答案→)A. to go
B. to going
C. having gone
D. go
解析： see, watch, look at ,feel, notice, observe, listen to, hear 等感官动词（短语）及make, let, have等役使动词后用不带to的不定式作宾补，但这类结构变成被动语态时，to
不可以省略另外，在感官动词后也可用现在分词作宾补，但其意义表示动作正在进行。如：I saw him running after the bus.(动作正进行)
