How do scientist usually test for one variable

Ecologist have observed a sharp increase of sulfur dioxide in the air . what might be a possible consequence of excessive sulfur dioxide in the air ? a. acid rain b. ozone layer depletion c.ozone layer hole d.increase in sea levels

Which of the follow describes a disadvantage secondary sources have compared to primary sources?

Which statement is true for bacteria a. bacteria have complex internal structures b. all bacteria are spiral in shape c. all bacteria cause disease in animals d. bacteria break down some foods