下列方程中，没有实数根的是（ ）
A：2x+3=0
B：x2﹣1=0
C：2x+1=1
D：x2+x+1=0
—This history book is of great value.
—Exactly. _____ can be enjoyed from it ______ you have a deep understanding of it, however.
在用猪心脏做灌流实验时，某同学将肺动脉和下腔静脉扎紧，然后向上腔静脉注水，水会从那条血管流出？（ ）
⑴____ A. vacation B. exam C. David D. favorite
⑵____ A. most B. post C. host D. cost
⑶____ A. seem B. feel C. coffee D. between
⑷____ A. however B. blow C. brown D. now
⑸____ A. clearly B. hear C. learn D. near
作为人体细胞的主要成分之一，约占体重的60%﹣70%，这种重要物质是（ ）
