Passage: "Carbon monoxide is a colorless, odorless gas produced when fuels such as gasoline burn incompletely. Because it cannot be seen or smelled, it is sometimes called the silent killer. Vehicles left running in enclosed spaces can produce dangerous levels of the gas." According to the passage, why is carbon monoxide called the silent killer?

1. It is extremely loud
2. It cannot be seen or smelled
3. It only forms at night
4. It is harmless in small amounts