一次性塑料袋曾被广泛使用。
2. 使用一次性塑料袋带来的问题。
3. 限制使用一次性塑料袋的意义。
Limiting the Use of Disposable Plastic Bags

A．Whether spacemen carry weapons.
B．How NASA trains its spacemen.
C．How spacesuits protect spacemen.
D．What spacemen eat and drink.

A．Basing them on science-fiction movies.
B．Adjusting them to different audiences.
C．Including interesting examples in them.
D．Focusing on the latest progress in space science.