7. 調光檯燈時亮時熄，在開關處或燈頭內發出聲響，其故障原因為(A)線路斷路 (B)線路短路 (C)接線鬆動 (D)電路過載

4. A thin stick with length d and mass m is rotated continuously at a rate of one revolution per minute about the axis through the point at 0.1d from one end. Which statement is correct? (A) The angular momentum is $73\pi md^{2}/9000 kg\cdot m^{2}/s$. (B) The angular momentum is 0. (C) The torque is $\pi md^{2}/30 N\cdot m$. (D) The torque is $2\pi md^{2} N\cdot m$. (E) None