19. Why should all measurements be done multiple times?

Increasing precision with multiple measurements

Explanation:

The more measurements you take (provided there is no problem with the clock!), the better your estimate will be. Taking multiple measurements also allows you to better estimate the uncertainty in your measurements by checking how reproducible the measurements are.

metals are good conductor of electricity. it has free electrons on their surface. according to electron pool theory, each atom present in a metal crystal losses all of its valence electron and as a result a pool of free is form. these pool of electrons are free to move and   conduct electricity easily   while making metal as good conductor.

the correct answer is a. the ability of electrons to flow throughout the metal.

if it's not useful, feel free to report for mistake in answer.

elements make up everything, and they combine to make compounds.

the answer is c