Because of vs Due to

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.

A have become an ongoing threat due to drought and booming population density, have increased
B have become an ongoing threat due to drought and booming population density, have been increasing
C has become an ongoing threat because of drought and booming population density, has increased
D have become an ongoing threat due to drought and booming population density, has increased
E have become an ongoing threat because of drought and booming population density, has increased

OA E

I got the question correct but I am not clear about the use of BECAUSE OF vs DUE TO

MGMAT explains that:

The phrase "due to drought . . ." is unidiomatic since "because of" (not "due to") should be used to modify the verb phrase "have become an ongoing threat."

Another source says:

1). Because - is a conjunction, used at the beginning of a clause, before a subject and verb.

We were late because it rained.
I'm happy because I met you.

2). Because of - is a two - word preposition, used before a noun or a pronoun.

We were late because of the rain.
I'm happy because of you.