主语从句练习题,求一些大神给解决一下~

主语从句练习题,求一些大神给解决一下~
11. It’s _____ he’ll be able to come. D
A. doubt whether B. doubtfulC. doubt it D. doubtful whether
12. It is _____ that the bridge has existed for more than three centuries . C
A. appeared B. risen C. said D. born
大家能帮帮忙吗,给个详细一点的解析,比如第一题为什么选D不选其他的选项呢,拜托~

11.选D.It's doubtful whether...是固定句式,意思是怀疑他是不是能够来.Doubt还有其他的用法一般是：1)It's no doubt that（表示对XX事情会怎样怎样毫不怀疑）;2)I doubt that...（表示我怀疑XXXX会怎样怎样）.doubt是动词/名词,doubtful是形容词.就像是It's important that...一样.Whether表示“是不是”的意思.
12.选C.It is said that意思是“据说”.A是出现,B是升起,D是出生.