An assembly line produces 90 cogs per hour until an initial order of 60 cogs is completed. The speed of the assembly line is then immediately decreased so that it only produces 60 cogs per hour until another 60 cogs are produced. What is the overall average output, in cogs per hour, for the assembly line during this whole time?	Choose the correct option:
A) 68
B) 69
C) 70
D) 71
E) 72

Rationale: "the time to produce the first 60 cogs is 60 / 90 = 2 / 3 hours . the time to produce the next 60 cogs is 60 / 60 = 1 hour . the average output is 120 cogs / ( 5 / 3 ) hours = 72 cogs per hour . the answer is e ."
Answer: E