For each of the elements below, there are only two naturally occurring isotopes. Using information in the periodic table, identify the pair in which the lighter isotope is the more abundant one.
a. 191 Ir and 193 Ir
b. 10 B and 11 B
c. 50 V and 51 V
d. 79 Br and 81 Br
e. 6 Li and 7 Li