判断下列段落的拓展方式The gap between business and teaching is widening. A secretary, an office clerk or a bank employee can earn monthly income as high as RMB 2,000, and when one is promoted to the position of a business executive or manager, the salary is even higher. But a college graduate with a master’s degree can only get a salary of RMB 1，000 per month for his teaching job. No wonder, college graduates are attracted into more lucrative fields.

A. Development by comparison and contrast
B. Development by definition
C. Development by example
Development by space