问题补充：
he was 12, Einstein had learned advanced mathematics by himself.
A. As soon as
B. At the time
C. By the time
D. During the time
答案：
A. As soon as
B. At the time
(答案→)C. By the time
D. During the time
解析：考查连词：A．As soon as一…就，B．At the time在…那个时候，C．By the time到…时候为止，D．During the time在那期间，四个选项BD不能做连词，A的意思不对，选C，句意：到他12岁为止，爱因斯坦已经自学了高等数学。选C。
