Betty is often seen ______the old man with his housework.

考查感官动词后接动词不定式的用法。在make, let, have等使役动词和see, watch, notice, hear, feel等感官动词后的复合宾语中，动词不定式不带to。当使役动词和感官动词变为被动语态时，必须还原to。故选B。

被动语态中的特殊情况掌握的不牢固。