Find a mistake in each sentence below

find a mistake in each sentence below  
1. the populate of the world has increased in modern time.
               A             B                  C               D
2. he is not enough old to join the army
         A                   B          C     D
3. everyone can learn how to swim, can they?
                      A              B      C             D
4. she has learnt english since three years, hasn't she?
                A                        B                 C              D
5. the price of the houses have doubled over the past few years
                   A                       B                    C                  D