Final answer:
The correct sentence is A, as "Eighteen inches" is a singular measurement and takes a singular verb when referring to a specific quantity of a submarine sandwich.
Step-by-step explanation:
The correct sentence between the options given is A: "Eighteen inches is more submarine sandwich than even the two of us can eat, I'm afraid." This sentence is correct because when talking about measurements, such as eighteen inches of a submarine sandwich, the measurement is considered as a single quantity or a unit and therefore takes a singular verb. Option B is incorrect because it treats "eighteen inches" as multiple individual units, which is not the case when referring to a specific length of something, like a submarine sandwich in this context.