Lenses are made to have different focal points. What two properties of a lens affect the focal length?
A. Where the object is located
O B. Whether the image is virtual or real
O C. The material the lens is made from
O D. The centers of curvature of the two sides of the lens

Which of the following statements is true? a. elements in the last period are radioactive. b. atomic weight is the same as atomic mass. c. elements in the same group have the same number of electron shells. d. atomic number equals the number of neutrons in the nucleus of an atom.

Which of the following signs of a chemical reaction are observed in the reaction of potassium with water? precipitate formed temperature change smell produced gas produced color change

Which of the following has wavelength longer than the wavelength of viable light? a) x rays b) gamma rays c) radios waves d) ultraviolet waves