When viewing the rhodospirillum and cyanobacteria it is necessary to use the technique and avoid trapping air by properly placing the .

Wet mount; cover slip

Explanation:

While viewing the rhodospirillum and cyanobacteria, the method which is used to visualize it under microscope is that it should be put on wet mount and cover slip should be put on it properly so that there is no bubble on the slide.

Bubbles in the slide makes the vision unclear and the bubbles appear in the microscope which makes the slide unuseful for visualization.

As morphology of the cell is important for the study of cell and if it is not clearly visible then the study cannot be performed effectively.

the correct answer is:

a population changes in response to a change in the environment.