下列估测中，最接近实际的是（　　）

A．中考规定书写笔芯的直径是0.5mm

B．常人的正常步行速度约3.2m/s

C．一盏家用白炽灯泡的正常工作电流约2A

D．物理书的质量约1.8kg

The little boy didn't know______.              [ ]

A. which hat is his                        

B. which hat his is              

C. which hat was his                      

D. which hat his was