I dont understand why my answer is wrong. I chose D but correct answer is E.

The number of acres destroyed by wildfires, which have become an ongoing threat due to drought and booming population density, have increased dramatically over the past several years, prompting major concern among local politicians.

have become an ongoing threat due to drought and booming population density, have increased
have become an ongoing threat due to drought and booming population density, have been increasing
has become an ongoing threat because of drought and booming population density, has increased
have become an ongoing threat due to drought and booming population density, has increased
have become an ongoing threat because of drought and booming population density, has increased

Can anyone help!?