In an experiment, a student uses a microphone together with a cathode-ray oscilloscope (CRO) to investigate a sound wave. The diagram shows the trace displayed on the CRO screen. The student expects a sinusoidal waveform to appear on the screen. Which changes should the student make to the time-base and the $y$-gain of the CRO so that the screen displays a continuous trace for one complete cycle of the waveform?
- Atime-base decrease, $y$-gain decrease
- Btime-base decrease, $y$-gain increase
- Ctime-base increase, $y$-gain decrease
- Dtime-base increase, $y$-gain increase