根据句意,写出括号内所给单词的适当形式空。
It’s ________ (possible) for dogs to fly. Stop daydreaming, Eddie.
Visiting _____________(Europe) countries is very expensive, but many people go there.
The poor boy’s parents have been ______________ (die) for years.
Few of us has the _______________ (know) of the world history in our class.