A student pulls a wooden box along a rough horizontal floor( without the block loosing contact with the floor) at constant speed by means of a force P as shown to the right. (W is weights of block, N is normal reaction on the block, f is force of friction on the block). which of the following must be true?
A. `Pgtf` and `NltW`
B. `Pgtf` and `N=W`
C. `P=f` and `NgtW`
D. `Pltf` and `N=W`
