7) During his lectures, Professor Kong likes to observe his class. In his first lecture, he observed a 95% attendance rate. He observed 92% in his second lecture, 88% in his third, and 83% in his forth. Which of the following concept is likely affecting the behaviors of Professor Kong's students? a. Elastic demand,- students can easily substitute the lecture with other activities. b. Deadweight social loss; there is a surplus because quantity demanded is less than quantity supplied. c. Quota; students stop attending lectures once they have acquired enough knowledge. d. Short run production curve; in the short run the professor and size of lecture hall cannot be adjusted, therefore only labor is adjusted. e. Diminishing marginal utility; one extra lecture's benefit is less than the benet from the previous lecture