What explains the fact that no machine is 100 percent efficient? a. friction transforms mechanical energy into thermal energy. b. thermal energy is transformed into mechanical energy by friction. c. potential energy can be transformed into kinetic energy. d. kinetic energy can be converted into mechanical energy.

Because a model would make the investigation easier to understand

The  grandmother cell  is a hypothetical  neuronthat represents a complex but specific concept or object.[1]  it activates when a person "sees, hears, or otherwise sensibly discriminates"[2]  a specific entity, such as his or her grandmother. the term was in use at least as early as 1966 amongst staff and students in the department of experimental psychology, university of cambridge, england. a similar concept, that of the  gnostic neuron, was proposed two years later

A.(nh4)(2co3)is the correct answer 

Green is technically every other color but green but the other wavelengths of the other colors get absorbed by the object and the wavelength of the color green gets reflected