Question: ① There are 1000 ping-pong balls in three boxes, including 500 in the red box, 200 in the blue box and 300 in the yellow box.

65,438+0,000 ping-pong balls are packed in three boxes, including 500 red boxes, 200 blue boxes and 300 yellow boxes. The overall individual difference is great, so stratified sampling can be used; Select 3 students from 20 students to participate in the symposium, and the overall number is small, so the lottery can be used.

To sum up, the problems and methods can be paired with ① Ⅲ and ② Ⅰ.

So choose B.