There are fewer and fewer trees.More and more people are always cutting down every year.

million B.
AThere are fewer and fewer trees.More and more people are always cutting down______every year.millions C.million of D

D

D 固定搭配：数词+ million of+名词表示几百万 或 millions of+名词表示成百万上千万， 意思差不多，只是搭配不同

选D million没有复数形式 number+million 就是几百万 但 millions of 是固定搭配 故选D

D，前面有基数词修饰，不用加s，没有就要加s和 of a million millions of 再看看别人怎么说的。