Why are black holes so difficult to detect?

Black holes are so dense that light cannot escape from them, and they are difficult to detect. Satellites above the Earth's atmosphere can detect bursts of high energy radiation given off by black holes. Dozens of stellar black holes have already been detected in the Milky Way, where there may be tens of millions more.

Explanation:

answer is: c. the experiment shows that the red substance experienced a chemical change.

this is chemical change (chemical reaction), because two new substances are formed (metalig liquid and gas), the atoms are rearranged and the reaction is followed by an energy change.

this is example of chemical decomposition.

chemical decomposition (analysis) is the separation of a single chemical compound into its two or more elemental parts or to simpler compounds.  

chemical decomposition is opposite of chemical synthesis.