Many environmentalists believe that the widespread planting of trees, along with the conservation of existing forests, would be one of the surest, easiest, and least expensive ways to begin to halt or even to reverse the buildup of carbon dioxide in the air.
(A) one of the surest, easiest, and least expensive ways to begin to halt or even to reverse
(B) one of the most sure, easy, and least expensive ways to begin a halt or even reverse
(C) one of the surest, easiest, and least expensive ways that would begin halting or even reversing
(D) a most sure, easy, and inexpensive way beginning the halting and even reversing of
(E) the most sure, easiest, and inexpensive way that would begin halting or even reversing

Why A and not C? Please explain