A composite block is formed by combining three transparent materials. The diagram shows a ray of light travelling through the composite block. Which list gives the three materials in order of the speeds of light in the materials, from slowest to fastest?
- A$1 \rightarrow 2 \rightarrow 3$
- B$1 \rightarrow 3 \rightarrow 2$
- C$2 \rightarrow 1 \rightarrow 3$
- D$2 \rightarrow 3 \rightarrow 1$