根据句意，用方框中所给短语的适当形式填空。 keep out, to start with, aim at, pro and con,  for instance 1. A sign outside a building says ' '. It is telling you not to enter the building. 2. , we only knew two people in London but we soon made some friends. 3. These advertisements are specifically __________ the young people. 4. We considered all the very carefully before deciding to buy a bigger house. 5. Several of his friends came :  Ben, Carol and Mike, _____________ .