The main reason for these is that the thermal conductivity of the modified SC in the Structure II is smaller and its specific heat capacity is larger than those of sand gravel in the Structure I .
The main reason for these后的系动词是不应该用are，应为我感觉that后面接了由and连接的两个并列句，视为两个缘由的话，应该用are吧
“主谓一致”是指，一个句子中谓语动词在人称和数上需要同主语维持一致。
遵循“语法一致”的原则，主语是单数（the main reason），谓语动词也要用单数（is）。宾语/表语的单复数跟谓语动词无关。
主、表两个定义不是指相同种类事物，不在一个层面上。主语是单数，表语可以是复数。比如：
Our only guide is the stars.
大家唯一的向导是星星。
Her only hope for the future is her children.
她对将来的唯一期望是她的孩子们。
The chief trouble in summer is the mosquitoes.
夏季的主要麻烦是蚊子。
Modern artsworks are a better choice for me.
现代艺术作品对我来讲是个更好的选择。
Bright comets are a beautiful sight.
明亮的彗星是一个漂亮的景象。
Newspapers are a good sourse of information.
报纸是信息很好的来源。
