Her favorite food is sandwich加复数吗？

His favorite food is sandwiches.
这里的复数，代表是各种各样的三明治，如果不加复数的，文法上不通，要加冠词或：
His favorite food is a sandwich.他钟爱的食物是一个三明治.（言下之意，他只需要一个三明治就够了）
His favorite food is the sandwich.他钟爱的食物是那种三明治（特指某种三明治）.