Which two ratios form a proportion?

Question

Responses
A 3 : 1 and 9 : 33 : 1 and 9 : 3
B 1 : 3 and 6 : 31 : 3 and 6 : 3
C 1 : 3 and 3 : 61 : 3 and 3 : 6
D 3 : 1 and 3 : 9