Complete this MC question by using S4 Economics and explain each options why it is correct and incorrect.

Paul is a university student, He can either spend his summer holiday taking two more subjects at the university or visiting Beijing on a self-financed exchange programme, Which of the following will raise his cost of taking two more subjects at the university? A. The fee for the exchange programme suddenly increases. B. The two subjects at the university are very boring. C. The air ticket price between Hong Kong and Beijing suddenly increases. D. A company suddenly offers him a summer job which has a higher value than the exchange programme