哪位请帮忙解道dB,dBm的选择题，多谢！

12-13

An amplifier has a gain of 9dB. Its input is fed with a sine wave within the bandwidth of the amplifier having a magnitude of 4dBm. The amplifier is perfectly matched at both input and output . What is the output voltage if the output is closed on a 50 ohm load?
a) 0.5Vrms
b) 1Vrms
c) 1.3Vrms
d) 2Vrms

我算答案是B

答案是B，能否给出不用计算器的求解方法呢？多谢！

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