A:随机取样、样方具有代表性、样本容量大是调查结果可靠的基本保证
B:树木A与B的差异属于遗传多样性,主要由基因突变和基因重组引起
C:树木D与G之间的差异属于物种多样性,它们之间存在着生殖隔离
D:若,则D=0.32,说明均匀度优,群落无分层现象
Gypsies
According to the linguistic and genetic researches, ancestors of the Gypsies left the land of India in the amount that did not go beyond 1,000 people. Today there are about 10 million Gypsies in the world. Some scientists believe that Gypsies were Egyptians.
Sumerians
Sumerians must be the mysterious people in the ancient world. We know nothing about their origin and nothing about the origin of their language. The Sumerians were rather advanced—they became the first in the Near East to use a wheel, to create an irrigation system and their unique writing. And besides, they were impressively good at maths and astronomy.
Maya
Some scientists believe that they are connected to legendary atlantes (男像柱), others suppose their ancestors were Egyptians. It's thought that the Maya created an efficient agricultural system and had deep astronomy knowledge. Their calendar was used by other people of Central America. Maya applied the hieroglyphic writing (象形文字) system. Now it seems that the Maya came from nowhere and disappeared nowhere.
寒潮是我国主要的气象灾害之一,它对农作物的危害最大季节是( )
生物武器的杀伤特点有 ( )①多数传染性强 ②发病快 ③污染面广 ④难以防治 ⑤无色无味