与what有关的宾语从句

与what有关的宾语从句
The manager came up to see ________
A. what was the matter B. what the matter was
C. what the matter is D. waht's the matter
这道题答案是A
解释是:因为本题主句用了过去式,故从句要用过去范畴的时态,所以排除C和D;又因为what在从句中充当主语,所以语序不变.
对于这一句"又因为what在从句中充当主语,所以语序不变"这一句我不大明白,希望回答这个问题的朋友能详细解释一下,最好再举几个例子,好的再加分~!
谢谢了!