单选题 It is essential that applicants whose first language is not English ______ sufficient linguistic competence to avoid any delay or difficulty in pursuing their studies.
A. had obtained B. would have obtained
C. should have obtained D. must obtain

【正确答案】 C

【答案解析】[解析] 虚拟语气
主句中如果含有祈使、命令、要求等语气，则从句中须使用should构成的虚拟语气。本句中的essential意为“必不可少的”，含有“强制要求”之意，故主语从句中须使用should构成的虚拟语气。