Which of the following substances is a compound?
a.
solid bronze (an alloy of copper and tin)
b.
a salt water solution
c.
solid iron metal (fe)
d.
hydrogen sulfide gas (h2s)

D. hydrogen sulfide gas (H2S)

Explanation:

A compound is composed of two or more elements chemically bound together.

In this case, hydrogen is chemically bounded with sulfur to form a gaseous compound called hydrogen sulfide.

the skeletons of most mammals reach a certain size and then stop growing. however, many animals, including some mammals, keep growing throughout their lives. kangaroos, for example, just keep growing and growing until they die. also, the longer an animal lives the greater the chance it might catch some deadly disease.