If r1, r2, and r3 have all different values, which resistors can you be sure have the same potential difference falling across them?

The resistors in parallel.

Explanation:

If you have 2, 3 or infinite resistors in parallel (that means when both of their terminals are respectively connected to each terminal of the others resistors), they all have the same potential difference even when they have different values. Of course that means if the resistor value increases, the current decreases since V=I.R, where V is a constant in the parallel resistors.

the answer is neither ball has a charge.

because if they were both positively charged, they will repel each other.

if they were positively and negatively charged, they will seek.

in this image, you can see solar radiation going through ocean water.

no image, but probably absorption