Is making a sandwich a chemical or physical change?

tell me why you chose this answer too!

What evidence supports the law of conservation of energy? a) mechanical energy is converted to chemical energy during photosynthesis. b) oxygen is made from the breakdown of carbon dioxide during photosynthesis. c)energy is absorbed by chlorophyll and becomes chemical energy during photosynthesis. d)the sun gives off light energy that is absorbed by plants.

Awave has a wavelength of 15 mm and a frequency of 6 hertz. what is its speed?

Ascientific idea becomes valid when it is what?