SAT语法,关于主语和宾语是否要同单复数（谓语是be动词时）

SAT语法,关于主语和宾语是否要同单复数（谓语是be动词时）
A)The disposal of nuclear fission's unwanted by-products is one of the problems.
B)Unwanted by-products of nuclear fission that need to be disposed of are one of the problems
解释里说是disposal和后面的one一致,但是我也看到过主语是复数,谓语宾语是单数的情况啊.
如果B要改是否要改成Unwanted by-products of nuclear fission that need to be disposed of are manyl of the problems
就是Unwanted by-products are problems.The unwanted by-product is one problem对不对？
比如The color is a problem of the nasty painting.