单选题 .  ①Astronomers studying a certain kind of supernova (exploding star) were surprised to find the supernovas were fainter than expected. ②Seeking explanations, they discounted the possibility that cosmic dust might be screening out some of the light, because it would filter out blue light more than red, causing the supernovas to appear redder than they really are. ③Also, unless spread very smoothly throughout space, the dust would introduce large variations in the measurements. ④Another possibility is gravitational lensing, the bending of light rays as they skirt galaxies en route. ⑤Such lensing occasionally causes brightening, but most often it contributes to the dimness of distant supernovas. ⑥Calculations show, however, that this effect becomes important only for sources more distant than the supernovas studied.45.  According to the passage, the astronomers rejected gravitational lensing as an explanation for their findings because ______