Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: 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?
the plasma membrane of the eukaryotic host
From the engulfing eukaryotic cell.