In Which of the Following Industries Would We Expect Collusion

In which of the following industries would we expect collusion to be most effective?

A) retail gasoline, where most gas is sold by a large number of small dealers
B) crude oil production, where most oil is sold by a small number of large sellers
C) housing construction, an industry dominated by local firms that produce unique products
D) soybean production, where there are large numbers of farmers in many countries