难－－请教ＮＮ

In rencent years cattle breeders have increasingly used crossbreeding ,in parat that their steers should acquie certain characteriatics and partly because crossbreeding is said to provide hubrid vigor.

partly to acquire certain characteristics in their steers根据ＯＧ说，这是正确答案，to aquire certain ....与and 后面的东西符合平行原则，可是我认为要符合也应该改成because ．．．，还有就是想请您帮我把这句翻译成汉语，特别是什么叫in their steers?

The period when the great painted caves at Lascaux and Altamira were occupied bu Upper Paleolithic people has been established by carbon-14dating,but what is much more difficult to determine are the reason for their decoration ,the use to which primitive people put the caves ,and the meaning of the magnificently depicted animals.ＯＧ说这句考平行,主语是the period,可是the period又怎么能被建立