请帮我讲讲这句话的考点!The tower being built is three times the height
请帮我讲讲这句话的考点! The tower being built is three times the height of the ancient one. 我在自学高一的英语. 我认为这句话的前部分应该是:The tower which is being built ,为什么直接用being built 也可以? 还有,若改成:The tower which is building 是否也可以?还是哪一种更好?