______系统的最大特点是：初级生产者十分分散，有的成灌木状或匍匐状生长，主要是一些有很强抗旱性的植物类群。
参考答案：荒漠生态
用高效液相色谱法测定一般护肤类化妆品中氢醌，已知标准溶液的出峰时间为3分51秒，如样品液也在3分51秒有一流出峰，在下面5种措施中，最不可取的措施为
A.改变色谱柱，如仍为阳性结果，即可报告含有氢醌
B.改变流动相，如仍为阳性结果，即可报告含有氢醌
C.测定该峰流出物的紫外吸收光谱，如仍为阳性结果，即可报告含有氢醌
D.测定该峰流出物的质谱，如仍为阳性结果，即可报告含有氢醌
E.直接出具含有氢醌的检验报告
Speaker A: I think tourism is a great way of helping the economies of developing countries.Speaker B: ().
A. Yes, I agree. I myself like nothing better than travel.
B. Maybe. Can you explain the advantage of developing tourism
C. Well, in my opinion, the governments of developing countries should create more job opportunities for their people.
D. Yes, it is. Without tourism, those countries would be much worse off.
