In humans, fungi can only infect the skin.

A. yeasts and molds: These are classified as fungi, not protists. They reproduce by spores and have distinct cellular structures different from protists.

B. algae and protozoa: Protists are a diverse group of mostly unicellular eukaryotes, and this category includes algae, which perform photosynthesis, and protozoa, which are animal-like and often motile.

C. helminths: Helminths are multicellular parasitic worms such as tapeworms and roundworms. They are not protists because they are much more complex organisms.

D. bacteria: Bacteria are prokaryotic microorganisms lacking a nucleus, while protists are eukaryotic and contain membrane-bound organelles.

E. viruses: Viruses are acellular entities that require a host to replicate, and they are not considered part of the protist kingdom.

A. An enzyme speeds up a chemical reaction in living organisms: Enzymes function as catalysts by lowering the activation energy needed for reactions, allowing biological processes to occur more quickly and efficiently.

B. An enzyme is a biological catalyst: Enzymes are proteins that serve as catalysts in living systems, regulating the speed of biochemical reactions without being permanently altered.

C. An enzyme reacts with the reactants and becomes part of the products: Enzymes do not become part of the products; they bind substrates temporarily, facilitate the reaction, and are released unchanged, allowing them to be reused multiple times.

D. An enzyme is specific to the chemical reaction it catalyzes: Enzymes have a highly specific active site that matches particular substrates, making them selective for certain reactions or molecular interactions.