1) ______ the theory of language underlying the natural approach
2) ______ the theory of learning underlying the natural approach
3) ______ one of the objectives of the natural approach
4) ______ one of the techniques of the natural approach
5) ______ one of the main features of the natural approach
a) problem solving activities
b) to help the students to develop basic communication skills
c) class time being devoted primarily to providing input for acquisition
d) communications as the primary function of language
e) the monitor model

简述地理环境中自然环境、经济环境、社会文化环境的含义。