Final answer:
The price increase of refrigerators by only $50 despite $100 external costs is due to consumer surplus.
Step-by-step explanation:
The increase in the market price of refrigerators by only $50, even though the external costs of pollution were $100, can be attributed to the concept of consumer surplus. Consumer surplus refers to the difference between what consumers are willing to pay for a product and the actual price they pay. In this case, even though the external costs pushed up the production cost, the market price increased by a lesser amount due to the willingness of consumers to pay higher prices for refrigerators.