let sb down和let down sb中，后者可以这么用么？

let sb down和let down sb中，后者可以这么用么？

没有后者这样的用法哦追问jim words hard at his lesson.He doesn't want to ____.
a. let down us b. let down his parents
c. let his parents sad d. let his parents up
这道题它给的答案是b，如果没这种用法，那么改选哪一个，还是4个都错？追答let down 放在一起用应该是be let down by sblet down 放在一起用应该是be let down by sblet down 放在一起用应该是be let down by sb追问也就是四个全错是吗？追答我是这样觉得的