题目

.  It is difficult for any of us to eat better, exercise more, and sleep enough,________ we know we should. A. because                    B. even if                            C. unless                      D. before 答案：B小麦叶肉细胞的核酸中，含有核苷酸的种类是（   ） A．8种           B．5种           C．4种               D．1种