Hobo doesn’t know .A. when to doB. what to doC. how to doD. where to do

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A. when to do
(答案→)B. what to do
C. how to do
D. where to do
解析：对于特殊疑词后接动词不定式，一般而言疑问副词+to do（vt） sth，或疑问副词+to do(vi)或疑问代词+to do （vt）,或疑问代词+vi+介词。本题符合第三种情况，故选B