Choose A if the statement applies to asexual reproduction, or S if it applies to sexual reproduction.

• It introduces genetic variation through the combination of alleles from two parents.
• It allows rapid population growth from a single parent organism in stable environments.

A student investigates the effect of temperature on the growth rate of Chlorella.

This is the student's method:

• Put a Chlorella suspension with nutrient medium in four different test tubes.
• Place the tubes in an incubator kept at a set temperature, under constant light.
• Repeat this process with test tubes kept at $10^{\circ }C$, $20^{\circ }C$, and $30^{\circ }C$.
• Leave the tubes for 48 hours under the same conditions.
• After 48 hours, measure the concentration of cells (in millions per mL) in each tube.

The table shows the student's results:

What is the independent variable in this investigation?

Identify the temperature where the mean concentration of cells is the same as the mode for the four test tubes.

• $10^{\circ }C$
• $20^{\circ }C$
• $30^{\circ }C$

The student thinks that there is a problem with their data at $30^{\circ }C$, which has made the calculated mean inaccurate. Explain how the student could improve their investigation to obtain a more accurate mean.

The student has concluded that extremely high temperatures slow down the growth rate of the algae. Explain how high temperatures slow the growth rate of Chlorella. Refer to enzymes and respiration in your answer.

The student also concluded that $20^{\circ }C$ is the optimum temperature for Chlorella growth. Their teacher states they need to extend the experiment to confirm this. Describe how the student should extend their experiment.