C

Explanation:
Strict product liability applies to both the sale and manufacture of defective and unreasonably dangerous products. For liability to attach, the product must be defective and unreasonably dangerous when it leaves the control of the seller or manufacturer, and the defendant must be in the chain of distribution (e.g., manufacturer, distributor, or retailer). Option A is correct because sellers, such as retailers, can be held liable for selling defective products. Option B is correct because manufacturers are liable for defects introduced during production. Therefore, Option C, which includes both situations, is the correct answer. Option D is incorrect because both situations apply.