The reams of studies and drafts documented by Gustave Eiffel indicate that the design of the Eiffel Tower generally emerged not in a rapid manner but developed painstakingly from multiple trial prototypes.

(A) emerged not in a rapid manner but developed painstakingly
(B) emerged not in a rapid manner but was painstakingly developed
(C) did not emerge in a rapid manner but developed painstakingly
(D) did not emerge in a rapid manner but had painstakingly developed
(E) did not emerge in a rapid manner but it was painstakingly developed

My Ques: difference between “developed painstakingly” and painstakingly developed?