sentence correction: Please explain why answer is c

According to Inter study, a nonprofit organization that studies health maintenance organizations (HMO's), they estimate that, in comparison to last year, when only 36 percent of the nation's 607 HMO's was profitable, this year 73 percent will be.

(A) they estimate that, in comparison to last year, when only 36 percent of the nation's 607 HMO's was profitable, this year 73 percent will be.

(B) compared to only 36 percent of the nation's 607 HMO's being profitable last year, they estimate 73 percent would be this year

(C) only 36 percent of the nation's 607 HMO's were profitable last year, it estimates that this year 73 percent will be

(D) it estimates 73 percent of the nation's 607 HMO's would be profitable this year, last year that was only 36 percent

(E) only 36 percent of the nation's 607 HMO's last year were profitable, whereas they estimate it this year to be 73 percent