OUAGADOUGOU, Burkina Faso, April 9 Four years of falling prices on the world market have West Africa's all-important cotton industry on the brink of collapse.World cotton prices, responsible for nearly 70 percent of impoverished Burkina Faso's cash exports and income for more than a quarter of its 13 million people, are now at the lowest since the Great Depression of the 1930s, the Times of London said.The market is said to be reeling because of what locals call the monster with three heads, namely, the dollar, low world prices and U.S. cotton subsidies. The United States' 25,000 cotton farmers receive subsidies totaling some $4 billion, reportedly allowing them to undercut their developing competitors.April 9