Here are some tips on using the Internet. Choose the best variant a, b or c. There is an example (0) at the beginning.

Talk with your (0) parents about rules for using the Internet. Always (1) follow these rules.
People from all over the world can (2) contact you. They are (3)
strangers to you. Don't forget about this when you are on the Net and you'll be (4) safe.
You shouldn't give out your (5) address, telephone number, parents' work address / telephone number without your parents' (6i permission.
Be very (7) cautious of any people (81 if they ask you to come to some place, or ask you to give money
You shouldn't write e-mail (9) messases all in CAPITAL LETTERS, people may think you are angry.

C=√(a^2+b^2) = √(8^2+14^2)≈16 см.