一个实数的平方总是正数______.
根据平方的非负性可得,一个实数的平方总是正数或0.
故答案为:或0.
Sometimes we _____ be busy shopping in a supermarket, for it's more convenient to purchase online nowadays. [ ]
A. needn't
B. mustn't
C. shouldn't
D. can't
评价维生素A营养状况的常用方法不包括()
A.血清维生素A水平
B.改进的相对剂量反应试验
C.视觉暗适应功能测定
D.血浆视黄醇结合蛋白
E.竞争受体结合试验