一、表示人的物主代词用my， our， your， his， her和their，指无生命的东西用its(但指国家时一般用she或her)，它们在句中作定语
二、名词型物主代词能作表语(It’s theirs)、主语(Mine is there)、宾语(I don’t like hers)，与of连用可以作定语(the food of theirs)。
1) A scientist bases its [A] work on hypotheses that [B] have been checked [C] through careful [D] experimentation.
2) Crude rubber is an [A] elastic(有弹性的) solid with a specific gravity of 0.911 and a refractive index(折射率) of 1.591， though it [B] composition varies with different latexes(橡浆) as well as [C] with the way it is prepared [D] at the plantation.
3) Manufacturers [A] of consumer goods [B] often change [C] the styles of them [D] products.
1) A错。 改为his.
2) B错。 改为its.
3) D错。 改为their.