Question

You must design a valid experiment to show that there are microorganisms in the air.  What would be your dependent variable?

Answer 1

growth of organisms in the broth i hope this helps

Answer 2

You can carry out a test , 
1- take a sample of the air in a test tube 
2-add limewater to the test tube 
3- result : if it turns milky/cloudy it has microorganisms 
4- reason: because every microorganism respires , and so it produces Carbon dioxide . 
and limewater is the test to show the presence of Carbon dioxide .