His parents used to ______ a living by growing vegetables.(A)making (B)earn(C)lead为什么不是A?

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His parents used to ______ a living by growing vegetables.
(A)making
(B)earn
(C)lead
为什么不是A?

为什么不是A
因为没有used to doing这种用法
be used to doing是习惯于的意思
它都没有be,怎么能选A呢?