单选题 It is well-knOWD that the more the old people use their brains the more mentally active they will be．
A. 这件事传开了，老人越多用脑，智力就越灵活。
B. 大家知道了，老人更多用脑，主动性更强。
C. 众所周知，老人用脑越多，智力就越活跃。
D. 众所周知，老人用脑越多，智力会更活跃。