prep1-77，并列的时候，可否省略助动词？

77。Scientists
            have
            found
            signs
            that moving
            water
            changed
            the
            chemical
            makeup
            of
            the
            surface
            of
            Mars
            in recent
            eras, therefore
                concluding
                that
                its
                crust
                is
                harboring
                up
                to
                three
                times
                as
                much
                water
                than
                was previously
            thought.

(A)
            eras, therefore
            concluding
            that
            its
            crust
            is
            harboring
            up
            to
            three
            times
            as
            much
            water
            than
            was

(B)
            eras, therefore
            concluding
            that
            the
            planet's
            crust
            harbors
            up
            to
            three
            times
            as
            much
            water
            or
            more
            as
            was

(C)
            eras
            and
            have
            therefore
            concluded
            that
            the
            planet's
            crust
            harbors
            up
            to
            three
            times
            as
            much
            water
            as

(D)
            eras
            and
            they
            have
            concluded, therefore, that
            its
            crust
            harbors
            up
            to
            three
            times
            as
            much
            water, or
            more, than

(E)
            eras
            and
            concluded, therefore, that
            the
            planet's
            crust
            is
            harboring
            up
            to
            three
            times
            more
            water
            as
            was

答案是C。

是一个have done, and have done结构，这里面第二个have，可以省略吗？

谢谢

在这里是不能省略的，因为省略改变了句子意思，成了完成时和一般时态的平行， 而这里没有明显的标志说要这两种时态。

我对划线部分倒是没什么疑问，但对最后一句:as much water as previously thought有个小小的疑问：这里第二个as后面补出was 和不不补有什么区别吗？