Comets can generate two types of tails as they move closer to the Sun and begin to heat up. A tail made up of ions (charged particles) is one type of tail. Which statement below about ion tails is the only one that is correct? 

a. An ion tail always points away from the Sun and disappears when the comet is at perihelion in its orbit. 

b. An ion tail always points toward the Sun and disappears when the comet is at perihelion in its orbit. 

c. An ion tail points back along the path of the comet's orbit and becomes longest and brightest just after perihelion. 

d. An ion tail always points away from the Sun and becomes longest and brightest when the comet is at perihelion in its orbit. 

e. An ion tail always points away from the Earth and never varies in its observed brightness.