two friends are talking to another person across the building the other person can hear them but not see them why is this?

In a voltaic cell, electrons flow from the (positive/negative) to the (positive/negative) terminal.

On a mild saturday morning while people are working inside, the furnace keeps the temperature inside the building at 23degreesc. at noon the furnace is turned off, and the people go home. the temperature outside is a constant 16degreesc for the rest of the afternoon. if the time constant for the building is 3 hr, when will the temperature inside the building reach 18degreesc? if some windows are left open and the time constant drops to 2 hr, when will the temperature inside reach 18degreesc?

3. which is not a primary color of light? blue red green yellow