Question

how can a scientist assess whether a pure niobium (nb) sample is responsible for contaminating the lab with radioactivity?
test the niobium sample to see whether it now contains other elements.
test the niobium sample for the presence of niobium oxide compounds
heat the niobium, and see if the level of radioactivity in the lab increases.
place the niobium under pressure, and see if the level of radioactivity in the lab increases

Explanation:

Radioactive substances emit radiation which can be detected by changes in the level of radioactivity in the surrounding area. Heating or placing a sample under pressure can cause changes in its physical - state or chemical - bonding situation and potentially release or increase the emission of radiation if it is radioactive. Testing for other elements or niobium oxide compounds does not directly assess the radioactivity of the niobium sample.

Answer:

C. Heat the niobium, and see if the level of radioactivity in the lab increases.
D. Place the niobium under pressure, and see if the level of radioactivity in the lab increases.