If you ________ at school, you _______ a college student now.

A．had studied hard; would have been

B．should have studied hard; should have been

C．had studied hard; would be

D．would study hard; must have been

7个月婴儿发热、呕吐、水样便，每天7～8次，已3天，于11月15日入院。查体：体温38.5℃，轻度脱水外观，咽部充血，心肺正常，肠鸣音亢进。大便镜检脂肪球(++)。外周血WBC5.6х10^9/L，N0.45，L0.55，最可能的诊断为()

A．上呼吸道感染

B．生理性腹泻

C．饮食性腹泻

D．病毒性肠炎

E．侵袭性细菌所致肠炎