某蛋白质由2条肽链、n个氨基酸组成，该蛋白质至少含有氧原子多少个[ ]
A．n+2
B．n-4
C．n+2
D．n+4
答案：C
Tommy is _____ only child in the family, so his parents make _____ fuss of him.
A．an; 不填
B．不填; the
C．不填; a
D．the; a
下列哪一项适宜病室湿度()
A．75～85%
B．50～60%
C．30～40%
D．65～75%
E．20～30%
