Scientific questions must be? a. based on scientific theories b. general c. biased d. testable

Scientific questions must be Testable :-)

Testable! because if you cant prove its not a theory 

The density of water is 1 g/cm cubed and in order for the pebble to sink its density has to be greater than the density of water and in order for the twig to float its density has to be less than the density of water