What impact does the empirical evidence suggest trade has on a country's growth rates?

a. The more trade a country engages in, the slower its rates of growth.
b. Trade increases a country's GDP level but has no effect on rates of growth.
c. It has a strong positive impact, though not as strong as investments in physical and human capital.
d. It is impossible to tell because higher growth may be either a cause or an effect of higher levels of trade.
e. Most of the empirical evidence is too ambiguous to draw hard conclusions about the impact of trade on growth.

Answer: c