Lets assume total distance = 2d.

we need to calculate 2d/4 = average speed for entire trip.

statement 1: tells us that bus travel 50MPH for first 2 hrs... clearly not sufficient.

statement 2: let 2v be the speed for first half, then v be the speed for second half.

d/2v + d/v = 4

d/4 = 2vsqure/3v

d/4 = 2/3 v.

as we don't know v. statement 2 is not sufficient.

combining st1 and 2. we can not calculate v. as 50 MPH given for first 2 hr and not first half of the journey.

Answer is E.

_________________

-------Analyze why option A in SC wrong-------