Several sources of electromagnetic waves in free space are shown below. 1. a radio wave transmitter 2. a source of X-rays 3. a $30\,\text{mm}$ wavelength radar transmitter 4. a light-emitting diode that emits red light Which sequence shows the sources arranged from the smallest to the largest wavelength, reading from left to right, of the waves produced by the sources?
- A$1 \rightarrow 3 \rightarrow 4 \rightarrow 2$
- B$2 \rightarrow 4 \rightarrow 1 \rightarrow 3$
- C$2 \rightarrow 4 \rightarrow 3 \rightarrow 1$
- D$3 \rightarrow 1 \rightarrow 4 \rightarrow 2$