In an experiment, a scientist decides to study the effect of exercise on cholesterol levels in people. He studies two set of people—those who exercise every day for an hour and those who don’t exercise at all. In this case, the statement that people who exercise for an hour may have lower cholesterol levels is . To test this statement, the scientist would measure cholesterol levels in exercisers and non-exercisers. The cholesterol levels would be
.

answer: the blood-brain barrier is a layer of specialized membranes and cell processes designed to keep undesirable substances from passing from the bloodstream into the precious tissues of the brain. while the blood-brain barrier does keep most bad things out of the central nervous system, it is not impermeable.

explanation:

b tumor cells have a cell wall instead of a membrane

explanation:

tumor cells different from normal cells because tumor cells have a cell wall instead of a membrane.

which

what is a measurement of the earth’s history divided into time periods?

> fossil record <

sedimentary rock

> geologic time scale <

radioactive elements