You are given a science question (easy-level) and four answer options (associated with "A", "B", "C", "D"). Your task is to find the correct answer based on scientific facts, knowledge, and reasoning. Do not generate anything else apart from one of the following characters: 'A', 'B, 'C', 'D'. There is only one correct answer for each question.
Q: According to the endosymbiotic theory, double-membrane mitochondria originated as a result of some larger eukaryotic cells with flexible membranes engulfing smaller prokaryotic cells. Which of these explains where the outer layer of the double membrane of the mitochondria originated?  
 (A) the plasma membrane of the eukaryotic host (B) the nuclear membrane of the eukaryotic host (C) the ribosomes of the engulfed prokaryotic cell (D) the lysosomes of the engulfed prokaryotic cell
A: 
A