Because the topic is to require defective products, 5C3 in A means that all three are defective products, and the defective products can be 1, 2,3, so A is wrong.

The answer is (5c1* 45c2+5c2 * 45 *1+5c3)/(50c3).

Or subtract the formula of 1-(45C3)/(50C3), that is, none of them are defective probability.

Hope to adopt.