What it functions as?

涂于德

Oil prices have risen by 32 percent since the start of the year, ______ a record US $57.65 a barrel on April 4.
A. have reached    B. reaching  
C. to reach      D. to be reaching

“B” is sure the key to this exercise. However, what does “reaching a record US $57.65 a barrel on April 4” function as? Please teach me in detail. I really can’t get it!