The number of swans in London's Hyde Park was decreasing at a rate of 8 per 100 swans in 2008. By 2010 the decrease was at a rate of 4 per 100 swans. If the rate declined by 20% from 2009 to 2010, by approximately what percent did the rate decline from 2008 to 2009?
A. 6% B.21% C.37.5% D. 41% E.49%​