名词性从句的语序问题

Question

1.I don't know what_____.
A.is the matter B.the matter is
2.I don't know ______.
A.what the reason can be B.what can the reason be
3.Do you know________?
A.who let the dog out B.who let out the dog
4._____is the most useful invention?
A.Which of this do you think B.Which of this to think
这些题都是关于语序的，没有干扰性的选项我就不写了。
那位高手可以解释一下？
万分感谢！
最后一题那个B项you写成to了~~

Answer 1

名词性从句的从句部分要用陈述句语序,即要把动词和名词换位置,有个例外就是what is matter,不用调整语序

Answer 2

陈述语序。
BAAA

Answer 3

b
b
a
a