the method of ratification being determined by Congress at the time of proposal.这为什么是being?

题目

the method of ratification being determined by Congress at the time of proposal.这为什么是being?
Before an amendment can take effect,it must be proposed to the states by a two-thirds vote of both houses of Congress or by a convention called by two-thirds of the states,and ratified by three-fourths of the states or by three-fourths of conventions thereof,the method of ratification being determined by Congress at the time of proposal.
这里是上下文,为什么不是is?而是being?

答案

the method of ratification being determined by Congress at the time of proposal.
这是独立主格结构.变成谓语句子结构是：AND therefore the method of ratification IS determined by Congress at the time of proposal.
--------需要有and作为“连接词”.