Give the correct form of verb or the word in the brackets

Give the correct form of verb or the word in the brackets:
1.Oh,no! I forgot ........... the door                     (lock)
2. Money can't buy .........                                 (happy)
3. English .............. as a first language in at least 75 continues around the world.              (use)
4. This river is extremely................                                (pollute)