7.The total force needed to drag a box at constant speed across a surface with coefficient of kinetic friction μ is least when the force is applied at an angle θ such that
(A) sinθ = μ
(B) cosθ=μ
(C) tan θ=μ
(D) cotθ=μ
(E) secθ =μ
答案：C
統計：A:0,B:0,C:0,D:0,E:1
難度：計算中