Is 9 is greater than 2,so 7.09 must be greater than 7.2.

According to that reasoning, you would say that  90  is
greater than  2,000,000 .

The value doesn't only depend on the digit.  It also depends
on the PLACE the digit is in.

When they're both in the same PLACE,  7.09  is greater than  7.02 .

But if you have  7.09  and  7.2 , the '2' is in a PLACE where any digit
is worth ten times as much as if it were in the place where the  '9' is. 
So  7.2  is greater than  7.09 .  It's actually about  1.6%  bigger.

Ithink the answer should be b.