THE PATIENT COULD NOT RECOVER UNLESS HE ……AN OPERATION.A-HAD UNDERG...

20- - BAI TAP TRAC NGHIEM CAU DIEU KIEN
BAI TAP TRAC NGHIEM CAU DIEU KIEN

20-the patient could not recover unless he

……

an operation.

A-had undergone

B-would undergo

C-underwentD-was undergoing