AM Water Resources - Critical Flow Problem

We've been battling this one in my corporate study group, no solution provided. Can anyone assist?

I got answer (C) through continuity equation  A1 x V1 = A2 x V2 

Watch for units conversion 

I got (c)

Hi,

An alternate method you might use is as follows:

Q = VA and V1A1 = V2A2   and A = D^2(PI)/4    D1=2" and D2 =1.5"

4.2 fps (2^2)(PI)/4 =V2 ((1.5^2)(PI)/4      now you cancel (PI)/4 or multiply each side by 4/(PI)

4.2(4) =  V2(1.5^2)

V2 = 16.8 cfs / (1.5 ft)^2      V2 = 7.467 fps or rounded to 7.5 fps

saves converting inches to feet, and you have to use your calculator for the last calculation.

I pretty sure when I took the FE exam I did it the hard way.