Cottonwood, Aspen, and Willow Trees Have Beige Flowers, with No

Cottonwood, aspen, and willow trees have beige flowers, with no petals, that appear before the tree's leaves are out in the spring; and they are dioecious. What does this indicate about these trees?

A) Their insect pollinators are specialists.
B) Early emerging insects are probably the pollinators.
C) Their pollen is dispersed by wind.
D) The trees are self-pollinating.