What Is One Known Difficulty Precluding Routine Use of Stem

What is one known difficulty precluding routine use of stem cell therapies?

A) They don't always work because sometimes even stem cells can be rejected by patients.
B) Many doctors find use of stem cells objectionable on religious or ethical grounds.
C) Producing stem cells requires hundreds of human eggs and the hormone treatments necessary to allow women to donate eggs can be quite risky.
D) Producing stem cells requires hundreds of human sperm and the hormone treatments necessary to allow men to donate can be quite risky.
E) The technology to extract stem cells from an early embryo does not exist yet.