Part a   
 
 the time constant (in ms) of the circuit is   
 
    20.  
 
 Part b   
 
 final value of the current (in mA) is   
 
    50.  
 
 Part c   
 
 the initial rate of rise of current (in A/s) is   
 
    2.5  
 
 Part d   
 
 time taken (in ms) to reach the final value of current is   
 
    20.