题目

WHEN you think of a typical American, whom do you picture? A pretty blond white American like Taylor Swift? Or a handsome black American like President Barack Obama or basketball star Kobe Bryant? In fact, there was a time when the average American looked like none of these people. Back in the year 1500, the average American was a brown-skinned hunter-gatherer who probably rode a horse and wore clothing made from animal skins. Today, these people－who usually identify themselves based on their individual tribes such as Iriquois, Apache and Navajo－are broadly referred to as “Nati下列对生态平衡的叙述中正确的是（ ）A．在生态系统中，各种生物的数量恒定不变B．在生态系统中，植物的数量总比草食动物多C．在生态系统中，各种生物的数量是不断变化的D．生态系统中，各种生物的数量和所占的比例总是维持相对恒定的状态