Stem cell research has been a controversial subject in past years. what is the reason for this?
a: cells for study are often taken from human embryos.
b: stem cell techniques will not accomplish the intended purpose.
c: replacement tissue will never be grown from a person's own stem cells.
d: the cells cannot be isolated.

The carbon cycle is best defined as a process in which a. carbon changes from inorganic forms to organic forms and back b. carbon is changed into other elements such as oxygen or nitrogen c. carbon is continually created from the sun’s energy by plants d. carbon is consumed and regenerated from other elements such as oxygen and nitrogen