Hel! M! Many scientists believe that stem cell research will one day lead to new treatments for diseases, but some people have ethical concerns about this research.

Which of the following characteristics of stem cell research is the most likely source of ethical controversy?

A. Stem cells naturally produce many different cell types.
B. Stem cell research requires specialized equipment.
C. Stem cells can replace damaged cells with healthy cells.
D. Stem cells are often extracted from human embryos.