Given below are two statements :
Statement $I$ : Down's syndrome is caused by the absence of one of the $X$-chromosomes.
Statement $II$ : Turner's syndrome is caused by the presence of an additional copy of the chromosomes.
In the light of the above statements, choose the correct answer from the options given below :

• A
  Both Statement $I$ and Statement $II$ are correct
• B
  Both Statement $I$ and Statement $II$ are incorrect
• C
  Statement $I$ is correct but Statement $II$ is incorrect
• D
  Statement $I$ is incorrect but Statement $II$ is correct