求助prep90

90. The
parachutelike membranes of Africa’s
scaly-tailed flying squirrels differ from those of the flying squirrels in the
family Sciuridae because they are attached to a cartilage rod at the elbow
instead of the wrist.

A. because they
are attached to a cartilage rod at the elbow instead of

B. because of
being attached to a cartilage rod at the elbow rather than at

C. in their
attachment to a cartilage rod at the elbow instead of being attached at

D. in that they
are attached to a cartilage rod at the elbow rather than at

解释差异，或说明有何特别之处时，用in that比because好。见OG208

because of 后只能接名词或短语，不能加doing之类的动名词。

这题我也觉得they有点问题，但是别的实在没法选。我觉得这里they应该指代前面一种松鼠。因为后面有rather than，都是以前以后做比较。