3. What is the main cause of acid rain? *
a. decrease in ozone levels
O b. burning of fossil fuels
c. changes in the atmosphere
O d. changes in the climate

What must happen before meiosis can begin? a. four gametes must be produced b. the four tetrads must be pulled apart c. dna doubles and produces sister chromatids d. the cell membrane pinches together

Acceleration is a direct result of a.) balanced forces b.) unbalanced forces c.) gravity d.) velocity

Which of these substances does not protect against invaders in the nonspecific immune response? a. saliva b. mucus c. tears d. urine