The displacement \[x\] of a particle varies with time \[t,x=a{{e}^{-\alpha t}}+b{{e}^{\beta t}}\], where \[a,\,b,\,\alpha \,\text{and }\beta \]are positive constants. The velocity of the particle will [CBSE PMT 2005]

• A.                         Go on decreasing with time                
• B.                         Be independent of \[\alpha \] and \[\beta \]            
• C.                       Drop to zero when \[\alpha =\beta \]              
• D.                         Go on increasing with time