Every human begins as a single, fertilized egg. After about five days, a hollow ball has formed that contains about 30 specialized cells called stem cells. Embryonic stem cells
can divide endlessly and give rise to every type of tissue in the body. Scientists hope
that someday stem cells will make it possible to repair or replace damaged tissues.
Embryonic stem cells used in research come from eggs that were fertilized in the
laboratory and donated for research. Adult stem cells used in research are found in
adult tissues. The use of embryonic stem cells is controversial because a human
embryo is destroyed to obtain these cells. However, currently adult stem cells cannot
be grown in the lab, and they occur in limited numbers in the body. Currently, large
numbers of stem cells are needed for stem cell therapy. How does this fact affect the
debate about the two sources of stem cells?

A. Embryonic stem cells are readily available but are not thought to be useful for stem cell therapy.

B. Adult stem cells are rare, and research needs to be done in order to find techniques to harvest more adult stem cells.

C. Embryonic stem cells are rare, and research needs to be done in order to find techniques to harvest more adult stem cells.

D. Somatic cells are destroyed during the collection of adult stem cells and there would be too much damage to justify the harvesting of the cells.