1997, 01, section 1, no. 17

In the 1980's the rate of increase or he minority population of the USA was nearly twice as fast as the 1970's.

c) twice what it was in the 1970's.

b) twice as fast as it was the 1970's.

official answer c) ; the reason is that: 'rate' can be followed 'fast' 'quick' 'slow', etc.???