The first question should be B and A. According to the last sentence, it is wrong. According to American history, it's C and D.

The second question should be B. The most powerful thing in America is Congress.

The third question should be C. The cabinet is only responsible to the parliament, and the king is only the head of state.

The above is just my personal understanding, I don't know if it's right? If not, please forgive me!