Passage: The B-45C was the first multiengine jet bomber in the world to be refueled in midair. It carried two 1,200 gal (4,542 l) wingtip fuel tanks, had a strengthened canopy, and an inflight refueling receptacle. The first B-45C was flown on May 3, 1949. Only ten were built, and the remaining 33 under construction were converted to RB-45Cs.

Question: How many were built?
ten