Two species of Amoeba $X$ and $Y$ were kept in fresh water and adapted to it. Species $X$ developed a contractile vacuole. When both were transferred to sea water and adapted,both $X$ and $Y$ lost their contractile vacuole. From these observations,we conclude that:

• A
  Both $X$ and $Y$ are marine species.
• B
  Species $Y$ is marine and $X$ is fresh water.
• C
  Species $X$ is marine and $Y$ is fresh water.
• D
  Both $X$ and $Y$ are fresh water.