A train starts from a station P with a uniform acceleration \[{{a}_{1}}\], for some distance and then goes with uniform retardation \[{{a}_{2}}\] for some more distance to come to rest at the station Q. The distance between the stations P and Q is 4 km and the train takes 4 minutes to complete this journey, then \[\frac{1}{{{a}_{1}}}+\frac{1}{{{a}_{2}}}=\]

• A. \[2\text{ }{{m}^{-1}}\text{ }{{s}^{2}}\]            
• B. \[4\text{ }{{m}^{-1}}\text{ }{{s}^{2}}\]
• C.        \[7.2\text{ }{{m}^{-1}}\text{ }{{s}^{2}}\]         
• D.        \[72\text{ }{{m}^{-1}}\text{ }{{s}^{2}}\]