Your Answer is

Correct Answer is C

Explanation

Assuming that a hamburger is x yuan, and a glass of soft drink is y yuan, then according to the meaning of the question, a system of equations can be formulated:

x+y=21, 2x+y=3.5

x=1.4, y =0.7

That is, the price of a soft drink is $0.7, the answer is C.