The sources of agglomeration economics can be classified into three broad categories: learning, matching, and sharing. Indicate which category best describes each of the following situations:

(a)  A student encounters a problem in solving the problem set. She asks her friend who is good at the course. The discussion with her friend helps her solve the problem set.

(b) All three major automobile companies in Detroit use motor vehicle tires pro- duced by the same factory, which is also located in the Detroit area.

(c)  Start-up companies in the IT industry concentrate in Silicon Valley because they can hire good engineers who fit their special demand for skills.