He must stay at home to look after his mother . She has been for a week.
A, ill . sick, B. sick ,ill C. ill ; ill D. sick ; illness
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解析：sick病态的，恶心的；ill生病的，邪恶的；illness病，疾病。句意：他必须呆在家照顾她生病的母亲。她已经生病一周了。结合语境可知前文描述的是他母亲的状态，ill当做生病讲时，只能用作表语，用作定语，意思是邪恶的。Illness是名词，结合语境可知前文用sick用作定语，下文ill用作表语，选B。