What natural barrier usually prevents two protons from combining?
A) the weak nuclear force
B) dark energy
C) electromagnetic repulsion
D) the strong nuclear force
E) gravity
C
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If each element in this circuit has a resistance of 12 
, what is the resistance of the entire circuit?
Which of the following statements is true of the solar wind?
A) It makes the Sun gain mass. B) It can be considered an extension of the corona. C) It makes the gases from the solar atmosphere flow along the magnetic fields that point inward. D) It refers to slow-moving atoms and ions that enter the solar corona.
The planet's orbital period is:
A) the time it takes it to rotate and have the same face toward us again. B) the time it takes to return to the same location in the sky, relative to the Sun. C) the time it takes for a satellite to orbit it. D) the time it takes for it to retrograde back to the same position as we pass it. E) the time its magnetic field takes to spin once.
A He-Ne laser (632.8 nm) is used to calibrate a grating. If the first-order maximum occurs at 20.5°, what is the grating constant (the distance between the slits)?
A) 0.905 ?m B) 1.81 ?m C) 2.20 ?m D) 3.62 ?m E) 4.52 ?m