6 of the fish are orange, the rest are yellow.
There are 18 fish in the tank, so the rest is 18-6=12.
12 of the fish are yellow.
The ratio of yellow fish to orange fish:
[tex]\frac{12}{6}=\frac{2 \times 6}{1 \times 6}=\frac{2}{1}=2:1[/tex]
The answer is C.
