Which of the following sentences involving a direct quotation is punctuated correctly? A. “The wall on which they are placed,” he explained, “must be at least four feet below the surface to avoid frost.” B. “The wall on which they are placed,” he explained. “Must be at least four feet below the surface to avoid frost.” C. The wall on which they are placed, he explained, must be at least four feet below the surface to avoid frost. D. “The wall on which they are placed, he explained, must be at least four feet below the surface to avoid frost.”