Many of them turned a deaf ear to his advice, _______ they knew it to be valuable.

A．as if

B．now that

C．even though

D．so that

乳牙充填后，继发龋是常见的并发症，下面哪一项是常见的原因()

A．乳牙牙龈 * * 位置低，容易食物嵌塞

B．乳牙釉质无机物含量高，儿童易食甜食，故易产酸脱矿

C．软化牙本质去除过多

D．乳牙备洞时不易达到抗力型和固位型，充填体易松动

E．乳牙牙髓血运少，抵抗力低