Bolivia has a population of three and a half million. About one tenth of the total population are white; one-fourth are mixed Indians and white; and more than half are full-blooded Indians. The government and the industries are under the control of the white people.   The Indians today live in much the same way as their ancestors. Most of them are farmers. Many work in the mines, for mining is the most important industry on Bolivia's. Of all the mining products, tin is the most important, which makes up three-fourths of all Bolivia's exports. The eastern slopes of the Andes, since there are very few roads there, are fertile but not highly developed. The two primary means of transportation are river boats and porters. East of the hill region is the great plains where tropical plants are grown. A serious problem for Bolivia is the transport of food from the warm regions, where it is grown, to the mountainous regions, where most of the people live.   Bolivia gained its independence from Spain about a hundred years ago. It has been slow in developing, but its rich resources promise a better future in it. The eastern slopes of the Andes is not highly developed because   _  . A) there are thick forests there B) there are very few roads there C) there are many fine mines there D) they are not fertile enough

B