提问一道GRE填空

提问一道GRE填空
Some paleontologists debate whether the diversity of
species has _______ since the Cambrian period,or whether imperfections in the
fossil record only suggest greater diversity today,while in actuality there
has been either ________ or decreased diversity.BE/CD
Blank (i)
Blank (ii)
A change
D stasis
B declined
E escalation
C increased
F discontinuity
不明白为何whether...,or...不表示对比,即greater diversity与空格能形成对比吗?另外请翻一下此句,