1. After examining the DNA of different organisms, which of the following did Erwin Chargaff conclude about the four bases?
a. A = T and C = G.
b. A = C = G = T.
c. A = C and G = T.
d. A + T = C + G.

a). A = T and C = G.

Explanation: Erwin Chargaff's rule states that in a double-stranded DNA molecule, the number of adenine is equal to the number of thymine and the number of cytosine is equal to the number of guanine. This means that there is a ratio purine to pyrimidine in a double-stranded DNA molecule is 1:1.

There are four bases found in the DNA: adenine, thymine, guanine and cytosine and according to the Watson and Crick base pairing rule, adenine pairs with thymine while guanine pairs with cytosine. Adenine and guanine are the purine bases while thymine and cytosine are pyrimidine bases.