The quality control manager selects 7 lightbulbs at random for inspection out of every 400 lightbulbs produced. A quantity of 20,000 lightbulbs is equal to
20,000
400
=50 batches of 400 lightbulbs. Therefore, at the rate of 7 lightbulbs per 400 lightbulbs produced, the quality control manager will inspect a total of 50×7=350 lightbulbs. Choices A, C, and D are incorrect and may result from calculation errors or misunderstanding of the proportional relationship.