Choose the sentence that is correctly punctuated.
A. The USDA is studying weeds that are resistant to common​ herbicides, or "'superweeds"'.
B. The USDA is studying weeds that are resistant to common​ herbicides, or 'superweeds'.
C. The USDA is studying weeds that are resistant to common​ herbicides, or "superweeds."
D. The USDA is studying weeds that are resistant to common​ herbicides, or "superweeds".