英语语法问题,请问星号中间“it” 指代什么?求高手指教,感激不尽

英语语法问题,请问星号中间“it” 指代什么?求高手指教,感激不尽Secondary stress could be divided into membrane and bending components,just as was done for primary stress,but after the removal of local membrane stress to the primary category,* it *appeared that all the remaining secondary stresses could be controlled by the same limit and this division was unnecessary.译文：正如主应力那样,副应力被分为薄膜和弯曲部分,但是,在将局部薄膜应力归到主应力类型后,就会有所有剩余的副应力被相同的限制控制,这种分划是没有必要的.