To answer this question, we need to consider the tense and subject-verb agreement in the sentence.
The sentence states that "items bought before 1940." Since "items" is plural, we need to use the plural form of the verb. Additionally, the sentence is referring to a past event, so we need to use the past tense.
Option A) "costs" - This option is incorrect because "costs" is the present tense form of the verb, which does not agree with the plural subject "items" and does not convey the past tense.
Option B) "cost" - This option is correct because "cost" is the past tense form of the verb, which agrees with the plural subject "items" and conveys the past tense.
Option C) "None of the above" - This option is incorrect because option B is the correct answer.
Option D) "Both a and b" - This option is incorrect because only option B is the correct answer.
The correct answer is option B because it agrees with the plural subject "items" and conveys the past tense.