Multiple choice Math. problem help please, how fast must the car go to keep 60 mph average speed

The problem is:
A race car is going around a square shaped track with one mile side. It makes it around the first three sides at 60, 60 and 30 mph respectively , how fast must it go through the last side to average 60 mph all the way around?
The choices are:
A. 90 mph
B. 60 mph
C. 120 mph
D. None of the above

Thank you.

Avg speed = total distance/ total time
60 mph = 1 mile/min
1 = 4/t
t = 4 minutes => average time to maintain 1 mile/min average speed
time to go around first 3 sides = 1 + 1 + 2 = 4 minutes
4 - 4 = 0 time left
hence: it is not possible to make average 60 mph all the way around(1st round) due to lack of time.
D) None of the above => answer