It is generally considered that the key a foreign language well is nothing but practice.A.

问题补充：

It is generally considered that the key a foreign language well is nothing but practice.

A. to learning

B. to learn

C. of learning

D. for learning

答案：

解析：考查介词。句中the key to……关键，to为介词，故选A。句意：人们普遍认为，学好外语的关键是练习。