How many g are equal to 45.7 mg?

The answer for #3 is c but i don't know why

Three cards with holes are arranged in a straight line. a light is shined through the first card’s hole and travels through all three cards. what does this tell you about light rays? a) that light is reflected b) that light is refractive c) that light travels in a straight line d) that light does not travel in a straight line

Which of the following units is not an official si unit? mole liter kilogram ampere