Sentence Correction

The National Transportation Safety Board has recommended the use of fail-safe mechanisms on airliner cargo door latches assuring the doors are properly closed before takeoff and to prevent them from popping open in flight.

(A) assuring the doors are properly closed
(B) for the assurance of proper closing
(C) assuring proper closure
(D) to assure closing the doors properly
(E) to assure that the doors are properly closed

[spoiler]A,B,C are clearly out because of faulty parallelism. What is wrong with D?[/spoiler]