Why do viruses need living hosts, such as bacteria or eukaryote cells?

When analyzing tree rings, what do scientists assume a thin ring indicates?

For a comparative or experimental investigation, scientists often make a testable about a scientific question, and then they test it in the investigation. a.control b.hypothesis c.procedure d.system

Which worm has eyespots that are used to detect light? a fluke b marine worm c planarian d tapeworm