计算：（-12）-18÷（-3）+（-5）．
原式=-12-（-6）+（-5）=-12+6-5=-11．
By the time Einstein was 14 years old, he ________ himself advanced mathematics.
A．has taught
B．taught
C．had taught
D．was teaching
一把钥匙开一把锁，现在有4把钥匙4把锁，最多开多少次可以确定哪把钥匙属于哪把锁？
