The instruments that are light in weight and small in volume are used in this space shuttle. 句中定语从句兼具下面哪种状语功能？

The instruments that are light in weight and small in volume are used in this space shuttle. 句中定语从句兼具下面哪种状语功能？
A.目的
B.转折
C.条件
D.原因
正确答案：原因