Every object has weight. But the center of its weight is not always in the middle of the object. If the top of an object is heavier than its bottom, it will fall over easily. We say that its center of gravity is high. But if the bottom is heavier, it won't fall easily. Then we say that its center of gravity is low.
    Look at the first picture. This empty box must fall over, because its center of gravity (C) is not above the line of its bottom (AB).
Now look at the second picture. There is some heavy iron in the bottom of this box. It will not fall, because its center of gravity is above its bottom.

①经过几____（A．jǐ  B．jī）个星期的努力，我终于完成了这项工作。

②就是这一句的鼓励，几____（A．jǐ  B．jī）乎改变了我的一生。

③茶几____（A．jǐ  B．jī）上放着一本书，散发着淡淡的墨香。

下列各项中，哪项不是造成生物多样性面临威胁的原因（    ）