SAT语法~选错Ancient documents (wrote on) vellum,a form of fine p

SAT语法~
选错
Ancient documents (wrote on) vellum,a form of fine parchment made of animal hides,must be stored under carefully controlled conditions because changes in humidity can be damaging.
括号里的是错的,why?
At least one course in statistics is recommended for (them) who plan to become journalists,because they will often need to evaluate whether information based on statistics is dependable.
括号里是错的,是不是要改成those?