One of the customs officers, who was watching the arrivals very ________, saw feathers ________ from under a woman’s coat.

I think the answer is right but I’m not ____ sure about it.

 Strong sandstorms are often _____ you cannot see the sun.

Chemical pollution will bring about a threat to agriculture and food chain, and ________ to human health.

Private cars really provide much convenience to people, and ___ they cause too many problems.