Read only variable can be assigned a value at the runtime. 
Where as const have to be assigned at the compile time only.

We can assign a value to a readonly variable at runtime 
till it is a unassined identifier, but once value is 
assigned, it cannot be changed.

In case of const we cannot assign a value at run time i.e. 
the value assigned to it at declaration time cannot be 
change

Readonly variable can be assigned in run time but constant
variables cnt assign at run time.constant variables is
assigned at compile time only

The difference between the readonly and const data members 
is that const requires you to initialize with the 
declaration, that is directly

all above answers are correct.But there is another point.

readonly variable can be assigned at declaration or at run 
time which is must be in constructor. Readonly value can't 
be modified.

Constant variable must be assigned at the time of 
declaration only ,which is can't be modified.

All the answers is right.
But the last one is very right.