A. The difference between the two types of hydrogen bonds is that the bond between adenine and thymine is a double hydrogen bond and the one between cytosine and guanine is a triple hydrogen bond.
B. The double bond can only occure between adenine and thymine because they are a perfect match, it is its specific base partner. The same happens with guanine and cytosine, only them can form the triple bond.
This is called complementary base pairing