Younger stars have more heavy elements because
a. old stars destroy heavy elements as they age.
b. young stars burn their nuclear fuels faster.
c. the heavy elements were made in previous generations of stars.
d. heavy elements haven't had time to settle to the core of these younger stars.
e. All of the other choices are correct.
c
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Indicate whether the statement is true or false
The work?
done by the force exerted by the spring on a mass attached to the end of the spring when the mass has displacement 
is
A. ?negative half the time, positive the other half of the time. B. ?always negative. C. ?always positive. D. ?negative more than it is positive.
In an x-ray diffraction experiment, using x-rays of wavelength ? = 0.434 × 10^?10 m, a first-order maximum occurred at 8° off the crystal plane. Find the distance d between crystal planes
a. 1.56E–10 m b. 2.19E–11 m c. 4.88E–10 m d. 1.70E–9 m e. 3.12E–10 m
In the proton-proton chain
a. no neutrinos are produced. b. energy is released because a helium nucleus has a greater mass than a hydrogen nucleus. c. no photons are produced. d. carbon serves as a catalyst for the nuclear reaction. e. energy is produced in the form of gamma rays and the velocity of the created nuclei.