Question 152860
(a) is not a function because x is assigned two y values. Remember that a function must pass the vertical line test in order to be called a function. (That is to say, you can not draw a vertical line between any two points). 

Moreover, if you are uncertain of a problem like this, make a quick plot of the points. If one point is above y=0 and the other is below y=0, and a less than b or b less than a for (a,y1) and (b,y2), it is indicative of a zero per the IVT.