A scientist tests a sample in the lab and finds
that it doesn’t contain DNA. Explain what the
scientists can conclude.

It is difficult to obtain a final conclusion with this information, but it is possible that the sample doesn't contain living organisms whose genetic material is composed of DNA. Another possible reason for this result is that the DNA extraction protocol used in the experiment is not suitable for this sample.

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:

✿   this landform has a high elevation and a high relief

✿   this landform is a mountain