What is the approximate chemical shift for the carbon labeled as "a" in a 13C NMR spectrum? The bond-line structure of a compound with a SMILES string of CC(CCCC)=O is shown. The second carbon of the parent chain, which corresponds to the carbonyl carbon, is labeled as a.
a. 30 ppm
b. 60 ppm
c. 300 ppm
d. 200 ppm