Lookup a Value and Display Column Header

aaa  bbb  ccc  D
1    5    9    # (aaa, bbb or ccc)
2    6    10
3    7    11
4    8    12

I am trying to find a formula (to go in D2) so that the number matches its column header (aaa, bbb or ccc). For example if I put 5 in cell D1, D2 displays bbb, 11 will display ccc and so on.