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28. Albert:_____________________Elissa:Oh, I see. I’ll add you later.(A) See you tomorrow. (B) Merry Christmas!(C) I just sent a friend request to you on Facebook. (D) I am full.

29. Anne:When will the football game start?Bill:In five minutes. ___________ We’re late.(A) Hurry up! (B) Time to go home.(C) Where? (D) It rains in summer.

30. Mark:Hi, Tony. What do you do to keep fit? Tony:____________________ Do you want to go with me? Mark:It sounds like a great idea!(A) It’s not your fault. (B) You are the painters.(C) I’m sure. (D) I swim seven times a week.

16. A plane electromagnetic wave in vacuum propagates in the $+\mathbf{x}$ direction with frequency $f = 900\mathrm{Mhz}$. The magnetic field oscillates perpendicular to $\mathbf{x}$ with amplitude $B_0 = 2.0uT$. What is the expression for electrical field $E(x,t)$? (Use $c = 3\times 10^{8}m / s$). (A) $6.67 \times 10^{-14} \cos(18.85x - 5.65 \times 10^{9}t)$ (B) $600 \cos(18.85x - 5.65 \times 10^{9}t)$ (C) $6.67 \times 10^{-1} \cos(0.33x - 5.65 \times 10^{9}t)$ (D) $600 \cos(0.33x - 5.65 \times 10^{9}t)$

18. The RC circuit shown in the figure $(\mathrm{R} = 100\Omega, \mathrm{C} = 10\mu \mathrm{F})$ is called a low-pass filter because it passes low-frequency AC signals with less attenuation than high-frequency signals. The frequency at which the voltage ratio $\mathrm{A} = \mathrm{v_{out}} / \mathrm{v_{in}}$ drops to $1 / \sqrt{2}$ of its maximum value is known as cutoff frequency. What is the cutoff frequency of this circuit?(A) $159.2\mathrm{Hz}$(B) $1\mathrm{kHz}$(C) $6.28\mathrm{kHz}$(D) $1.59\mathrm{MHz}$(E) $10\mathrm{MHz}$