it is the room （which president work in）
其中is 是谓语,括号内为定语从句
the room which is the president work in还是不对
the room （which the president work in）是个名词,括号内为修饰这个名词的定语
注意,重点是定语从句只能有一个谓语
把它还原成两句句子
it is the room .President work in the room.
你看,第二个句子中的谓语是work