Final answer:
If pollution kills off the majority of plankton, the population of blue whales would decrease.
Step-by-step explanation:
If pollution kills off the majority of plankton, the population of blue whales would decrease. Plankton is the primary food source for blue whales, so a decline in plankton population would result in a scarcity of food for blue whales. Without enough food, blue whales would struggle to survive and reproduce, leading to a decrease in their population size.