contribution,contributions

问题描述:

contribution,contributions
同一篇文章中有的地方用contribution,有的地方用contributions,请分析下,
I would like to thank Mr Tan Tat Chu,Paul Tan,Ms Yuan Jiang and Ian Martin at Thomson Learning Asia for their contribution to the project.
...
Additional thanks to Robert Lee for contributing his skill as singer/musician to the series,and to Zhang Linlin,Lu Xiqin and Guo Juan for their valuable contributions.

我是这么想的,第一句中的contribution是名词贡献,但没有量化它的意思,感谢这些人对项目的贡献,但后面一句是说这些人有价值的贡献,形容词描述一个量化的概念...