How many core biosafety work practices (or do's and don'ts) are there, according to the National Research Council’s book entitled, Prudent Practices for the Handling and Disposal of Infectious Materials?

seven

Explanation:

There are seven biosafety practices which should be followed during the Handling and Disposal of Infectious Materials. These biosafety practices were made by the scientists in order to reduce the chances of contamination of worker who deals with the handling and disposal of Infectious or toxic materials. If these seven biosafety practices were not followed by the workers so there is high chances of infection which leads to their death.

the dark variety, as they are not visible on the dark tree barks

explanation: