再請教一題

When the prime lending rates went up in 1987, economists determined they would cause interest rates to rise and then decline over the ensuing five-year period.

(A) they would cause interest rates to rise and then decline

(B) they would mean that interest rates would rise and then decline

(C) that they will cause interest rates to rise and then decline

(D) that the increase would cause interest rates to rise and then decline

(E) that the increase would cause interest rates’ rising and subsequent declining

大全上的答案是C

是否是因為determine後接that從句需用肯定語氣will之故??