Which of the following is true about the function f(x)=10-x^3

Question

a) f is 1-1 since a vertical lines crosses the graph at one point
b) f is not 1-1 since a vertical line crosses the graph at more than one point
c) f is 1-1 since a horizontal line crosses the graph at one point
d) f is not 1-1 since a horizontal line crosses the graph at more than one point

Answer 1

well, only a and c are true
which makes the difference? (f is a function)