Researchers have long noted strange grooves near the gum lines on

The anthropologist's hypothesis is the following:

the early humans used grass stalks [for tooth picking]

The support for that hypothesis is the following:

[The grass stalks] unlike wood, contain abrasive silica, a substance that would facilitate the development of the grooves

The correct answer must be "useful to the evaluation of the anthropologist's hypothesis."

Since the answer choices are statements, the correct answer will simply weaken or strengthen the argument.

A. The dental remains that have the type of grooves in question almost as commonly show signs of tooth decay as do the remains that lack the grooves.

This choice has no effect on the argument. The fact that the dental remains show signs of tooth decay does not indicate anything about what caused the grooves or whether the early humans used grass stalks for tooth picking. After all, dental decay could occur regardless of whether they did or didn't use grass stalks for tooth picking.

Eliminate.

B. Dental remains of some of the early humans without the grooves have been found at places where the available grass could have been suitable for tooth picking during their lifetimes.

The fact that grass suitable for tooth picking may have existed where early humans' teeth did not have the grooves doesn't tell us whether the grooves were caused by use of grass for cleaning teeth. After all, it's not the case that all early humans must have used the available resources in the same way. It's likely that some early humans didn't use grass to clean their teeth.

Eliminate.

C. Unlike grass stalks, few modern-day toothpicks contain significant amounts of abrasive silica.

This information has already been provided by the passage. The passage indicates that grass stalks are "unlike" modern toothpicks in that grass stalks contain silica. Information already provided cannot have any new effect on the argument.

Eliminate.

D. Abrasive silica derived from grasses and other, similar plants could be useful in the removal of cavity-causing plaque from humans' teeth.

This choice has no effect on the argument. The fact that abrasive silica derived from grasses and other, similar plants could be useful in the removal of cavity-causing plaque doesn't mean that early humans used abrasive silica or grass stalks containing abrasive silica to clean their teeth.

Eliminate.

E. The grooves occur on the teeth of some early humans whose remains were found at places where no grass suitable for tooth picking would have been obtainable during their lifetimes.

This choice weakens the argument. We see that the effect "grooves occur on the teeth" exists where the presumed cause "grass suitable for tooth picking" did not exist. This information indicates that the use of grass for tooth picking was not the cause of the grooves since the grooves appear to have occurred even without the grass.

The correct answer is