FORTUNATELY, THE HOSPITAL'S NEW AIR-CONDITIONING SYSTEM________ WHEN THE FIRST HEAT WAVE OF THE SUMMER ARRIVED

25. Fortunately, the hospital's new air-conditioning system________ when the first heat wave of the summer arrived.A. Choose the one word or phrase -a, b, c, or d - that best completes the a. had installed b. installedsentences, or substitutes for the underlined word or phrase.c. had been installed d. had been installing