D
Organization president: The stationery and envelopes used in all of the mailings from our national headquarters are made from recycled paper, and we never put anything but letters in the envelopes. When the envelopes have windows, these windows are also made from recycled material. Ttherefore the envelopes, and thus these mailings, are completely recyclable. Which one of the following is an assumption on which the organization president's argument depends?
A: The envelopes and stationery used in the organization's are always recycled.
B: The mailings from the organization's national headquarters always use envelopes that have windows.
C: The organization sends mailings only from its national headquarters.
D: The envelope windows made from recycled material are recyclable.