The country of Yipi can raise its productivity by investing more in:

A) both human and physical capital.
B) human capital only.
C) physical capital only.
D) stable foreign economies.
E) stocks and bonds rather than physical assets.

Investing in both human and physical capital can lead to an increase in productivity. Human capital refers to investment in education, training, and healthcare for the country's population, while physical capital refers to investment in infrastructure and technology. Both of these types of investments can improve productivity and contribute to economic growth. Factors such as stable foreign economies and investments in stocks and bonds may also have an impact, but they are not as directly related to productivity as investments in human and physical capital.