what was to buy
what to buy
which to buy
which to be bought
“疑问词+不定式”结构可以做 know/wonder 等动词的宾语。本题 what to buy 和 which to buy 都符合语法结构。但 which to buy（买哪一个）在有选择范围的情况下使用，而 what to buy（买什么）在无选择范围的情况下使用。故选B。
