A box slides 4m north on a smooth horizontal floor while three constant forces act on it:
(i) 20N due north,
(ii) 15N due east,
(iii) 50N vertically downward.
Which one of the following statements about the work done by these forces during the slide is correct?
a) Forces (i) and (ii) do positive work; force (iii) does negative work.
b) Only force (i) does positive work; forces (ii) and (iii) do zero work.
c) All three forces do positive work because each has non-zero magnitude.
d) Only force (i) does positive work; forces (ii) and (iii) do negative work.