(2013·安徽，21)From space, the earth looks blue.This is ________about seventy－one percent of its surface is covered by water.

A．why

B．how

C．because

D．whether

教师对包括职业技能在内的人类精神财富加以吸收和总结，然后传授给学生，使他们在较短的时间内能参加社会实践活动。