Critical Reasoning

Between 2004 and 2007, the number of robbery cases in Chef County reached an average of 3 reported robberies
per month! Immediate measures had to be taken and Sheriff Sandberg was brought on board in December 2007.
Since then, in the following three years, the average has lowered to just 1.2 reported robberies per month!
Clearly, the number of robbery cases in the town has significantly reduced because of the new Sheriff's approach.

The argument assumes which one of the following?

A. The figure representing robberies reported as a percentage of total robberies remained fairly constant from 2004 through 2011.
B. 1.2 robberies per month is the lowest figure reported since the year 2000.
C. The overall crime rate in the entire state did not go down drastically during 2008-11.
D. In June 2008, the definition of 'robbery' underwent a change in the police terminology because of which many cases which would have been entire termed as robberies were not termed so now.
E. The support staff to the Sheriff remained duCring the period 2004-2011

What's the best approach to determine the answer? Can any experts help?