Which of the following is an observation and not an inference? a. a cup with steam coming from it will be hot to the touch. b.dark clouds in the sky will mean rain later in the afternoon. c.bubbles form when two chemicals are mixed together. d.a plastic bag moving along the ground is being pushed by the wind.

the answer is b.  

manipulated by changes to the independent variable.

i do not get it   at all

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.