Answer & Solution
Answer: Option B
Solution:
$$\eqalign{
& {\text{Let}}\,{\text{us}}\,{\text{name}}\,{\text{the}}\,{\text{trains}}\,{\text{as}}\,{\text{A}}\,{\text{and}}\,{\text{B}}{\text{.}}\,{\text{Then}}, \cr
& \left( {{\text{A's}}\,{\text{speed}}} \right):\left( {{\text{B's}}\,{\text{speed}}} \right) \cr
& = \sqrt b :\sqrt a \cr
& = \sqrt {16} :\sqrt 9 \cr
& = 4:3\, \cr} $$
