Ureters do not have valves at their ends opening into the urinary bladder, yet urine does not flow back into them on contraction of the bladder. What is the reason?

The question given below consist of an assertion (A) and a reason ( R). Use the following key to choose the appropriate answer to these questions from the codes (a), (b), ( c) and (d) given below: Both A and R are true and R is the correct explanation of A. Both A and R are true but R is NOT the correct explanation of A. A is true but R is false. A is false and R is also false. Assertion: Insulators do not follow electric current to flow through them. Reason:Insulators have large number of free electrons.