下列我国古代科技成就中,两项都属于同一领域的是[ ]
A.《千金方》和《本草纲目》
B.《水经注》和《农政全书》
C.《缀术》和《天工开物》
D.《齐民要术》和《梦溪笔谈》
答案:A
等差数列{an}前n项和满足S20=S40,下列结论正确的是( )
A.S30 是Sn中最大值
B.S30 是Sn中最小值
C.S30=0
D.S60=0
How much time do you often spend _________ English every day?
A.to practice speaking
B.practicing to speak
C.on practicing to speak
D.practicing speaking