A traveler has 3 ways to go from city A to B and 4 ways to go from B to C. How many ways are there to go from A to C?

Correct answer: 12

The governing concept is the multiplication principle for consecutive stages. Every route from A to C consists of one choice for travelling from A to B followed by one choice for travelling from B to C. For each of the 3 first-stage routes, there are 4 possible second-stage routes. Thus the total is 3 × 4 = 12, so option D is correct. Adding 3 and 4 would treat the two stages as alternatives, but both stages are required in the same journey. The value 16 uses the wrong number of choices, and 9 has no corresponding product or valid counting interpretation. The routes are counted as ordered pairs: first-stage route, second-stage route. Assuming the route choices are available independently, the multiplication rule gives the complete count without omission or repetition.