He always tells funny things and always ______.        [      ]

A. makes Lucy to laugh

B. makes Lucy laughing

C. makes Lucy laugh

D. makes Lucy laughs

计算题：已知圆钢两端沿轴线分别作用拉力F=40KN，圆钢的直径为20mm，圆钢材料的许用应力[σ]＝160MPa，试问圆钢截面上的应力，圆钢是否安全。