12. a(n) _ is an underground reservoir of water that is over-
lain by impermeable rock units.
a. aquiclude
c. confined aquifer
b. aquifer
d. unconfined aquifer

b. Aquifer

Explanation:

An aquifer is a body of saturated rock through which water can move quickly. An aquifer must be both penetrable and porous. It includes such rock types as sandstone, conglomerate, fractured limestone, and unconsolidated gravel and sand. Fractured volcanic rocks, such as columnar basalts, can also make suitable aquifers. The rubble zones between volcanic flows are generally both porous and permeable and make excellent aquifers. Rocks such as granite and schist are generally poor aquifers because they have very low porosity.

step 1: initiation

initiation is the beginning of transcription. it occurs when the enzyme rna polymerase binds to a region of a gene called the promoter. this signals the dna to unwind so the enzyme can ‘‘read’’ the bases in one of the dna strands. the enzyme is now ready to make a strand of mrna with a complementary sequence of bases.

step 2: elongation

elongation is the addition of nucleotides to the mrna strand. rna polymerase reads the unwound dna strand and builds the mrna molecule, using complementary base pairs. there is a brief time during this process when the newly formed rna is bound to the unwound dna. during this process, an adenine (a) in the dna binds to an uracil (u) in the rna.

step 3: termination

termination is the ending of transcription, and occurs when rna polymerase crosses a stop (termination) sequence in the gene. the mrna strand is complete, and it detaches from dna.

i think the answer is b.