Because the results are consistent
<span>The experimenters should make sure they are using proper clothing and shoes while properly coverings the exposed skin areas. Apart from that they should take proper measures to protect their eyes with proper safety goggles. If using contact lenses rather use eye glasses instead of that.</span>
The independent (or manipulated)variable is something that the experimenter purposely changes or varies over the course of the investigation. The dependent (orresponding) variable is the one that is observed and likely changes in response to the independentvariable.
Answer: pressure changes
Valves of the heart open and close due to pressure changes
Explanation:
The veins are the blood vessels possessing valves. And because they carry blood away from the heart, the valves helps to prevent back flow of blood due to the low pressure of blood found in the veins.
Thus, Valves of the heart open and close due to pressure changes