Maria told Tom ----- she had looked for Susan but ---- she c

"but"连接的两句句子都是told的从句,当一个动词后有多个宾语从句时,连词that不能省略.
句子结构
Maria told Tom【 that （she had looked for Susan ）but （that she could not find her）】.
（）中的两句句子并列,并用but连接组成复句,做told的宾语,通过that与主句Maria told Tom连接