The cakes are delicious.I’d like to have ---- third one as -

The cakes are delicious.I’d like to have ____ third one as _ second one I ate was too small.
A.the,the B.a,the C.the,a D.a,a
为什么不选B 定冠词的用法是序数词前都加the,那为啥这个就不行
我说反了
应该选B
我选的是C
但不懂