Rearrange the following parts to form a coherent paragraph:
a. over much of the Sahara, these formations were covered by Mesozoic deposits,
b. the Sahara sits atop the African Shield,
c. and many of the important regional aquifers are identified with them,
d. which is composed of heavily folded and denuded Precambrian rocks,
e. have remained horizontal and relatively unaltered,
f. because of the stability of the shield, subsequently deposited Paleozoic formations,
Which of the following will be the third statement after rearrangement?