“鸟类的黑手党”﹣﹣杜鹃，将自己的卵产到其它小鸟的巢中，小鸟辛勤地为杜鹃孵卵并精心喂食雏鸟．对小鸟的这种行为解释正确的是（    ）

①是先天性行为   ②是由环境因素决定的   ③是学习行为   ④是由遗传物质决定的

    The Great Pyramid(金字塔)was built nearly 5,000 years ago for a king called Khufu. It is on the west bank of the Nile River. In fact, all the pyramids along the Nile River are on its west bank. The ancient Egyptians thought of the rising of the sun as the beginning of life and the setting of the sun as the end of life. This is why their bodies were buried on the west of the Nile.

    It is very hard to realize just how big the Great pyramid is. It has over 2,000,000 pieces of stones. Most of these big stones weigh two and a half tons each. Some even weigh fifteen tons. Without machinery, the ancient Egyptians cut and moved and lifted each of these stones. Almost all the stones came from the east bank of the Nile, and they were taken across the river in boat. It took more than 100,000 people twenty years to build the Great Pyramid.

    The Great Pyramid is over 450 feet high today. Each of the sides of the Pyramid is 755 feet long. It takes about twenty minutes to walk all the way around it.