1.____the sound of the knocking on the door,he rose and went to open it.
A.On B.Heard C.At D.To
2.She seemed pleased when she met her cousin___the first time
A.at B.in C.for D.by
3.Taking him____the famous movie star,they gave him a hearty welcome.
A.upon B.to C.for D.into
4.I owe thanks__you ____your help.
A.to;for B.for;for Cto;of D.for;to
5.Red flags stand out brightly___the blue sky.
A.against B.by C.under D.above
希望高手帮着解答一下，并帮忙分析一下

C C C A A
2. for the first time...第一次。这是固定用法
3. take someone/sth. for ...把某人（物）误认为人（物）。
4. owe...to...对...感激/将...归功于。 for在此表示原因。
5. against 映衬之意。