—Why not take ____ umbrella with the sign “Made in China”? It's ____ useful one.—What ____ good advice.
A:an; an; a
B:an; a; \
C:a; an; a
D:\; an; a
保护生物的____环境,保护生态系统的____,是保护生物多样性的根本措施.
①b2﹣4ac<0,②abc<0,③4a+2b+c=1,④a﹣b+c>10中,判断正确的有( )