The Asian elephant walks with at least two, and sometimes three, feet on the ground at all times. Even though it can accelerate, it does so merely by taking quicker and longer steps. So the Asian elephant does not actually run. The conclusion drawn above follows logically if which one of the following is assumed? ['To run, an animal must have all of its feet off the ground at once.', 'If an animal cannot accelerate, then it cannot run.', 'It is unusual for a four-legged animal to keep three feet on the ground while walking.', 'The Asian elephant can walk as quickly as some animals run.']