zero-based budgeting

A definition of zero?based budgeting is set out below, with two blank sections.
“Zero?based budgeting: a method of budgeting which requires each cost element
___________, as though the activities to which the budget relates _______________.”
Which combination of two phrases correctly completes the definition?

Blank 1 Blank 2
A to be specifically justified could be out?sourced to an external supplier
B to be set at zero could be out?sourced to an external supplier
C to be specifically justified were being undertaken for the first time
D to be set at zero were being undertaken for the first time

sir the answer is C. But i am wondering why D is wrong?