The common denominator of 3/4 and 9/10

The common denominator would be 20 , 3/4 would turn into 15/20 and 9/10 would be 18/20

To common denominator would be 20, since 4 and 10 can both become 20 after multiplying themselves with other numbers, in this case 5, and 2. 
So 3/4 would be 15/20, and 9/10 would become 18/20.

The answer is the third choice