WHO AM I ?

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Across
  1. 3. I am green. I can live both on land and in water.
  2. 6. I have got hump on my back and I live on the sand.
  3. 7. I am long. I don't have arms and legs.
Down
  1. 1. I am black and white but not an old TV.
  2. 2. I have a trunk but I am not a tree.
  3. 4. I have a horn but does not beep.
  4. 5. I am the tallest land animal.