Rearrange the following five sentences (1), (2), (3), (4) and (5) in the proper sequence to form a meaningful paragraph; then answer the questions given below them.
(1) For instance, if we measure the room temperature continuously and plot its graph with time on X-axis and temperature on the Y-axis, we get a continuous waveform, which is an analog signal. Analog is always continuous.
(2) The absence or presence of something can be used to plot a digital signal.
(3) An analog signal is a continuously varying signal, similar to a sinusoidal waveform.
(4) Any signal can be classified into one of the two types: analog and digital.
(5) In contrast, a digital signal takes the form of pulses, where we have something or nothing.
Which of the following should be the THIRD sentence after rearrangement?