prep163

163. Unlike other Mayan cities, Cancun's
commercial power throughout the lowlands seems to be from using its strategic
position at the foot of the highlands, which were a source of jade, obsidian,
and other valuable commodities.

A. Cancun's commercial power throughout the
lowlands seems to be from using its strategic position at the foot of the
highlands, which were a source of jade, obsidian, and other valuable
commodities

B. Cancun's commercial power throughout the
lowlands seems to have come from using its strategic position at the foot of
the highlands, a source of jade, obsidian, and other valuable commodities

C. the commercial power of Cancun
throughout the lowlands seemed to have come from using its strategic position
at the foot of the highlands, a source of jade, obsidian, and other valuable
commodities

D. Cancun seemed
to be using its strategic position at the foot of the highlands, which was a
source of jade, obsidian, and other valuable commodities, in becoming a
commercial power throughout the lowlands

E. Cancun seems to have used its strategic
position at the foot of the highlands, a source of jade, obsidian,
            and other
valuable commodities, to become a commercial power throughout the lowlands

Cancun seems to have used its strategic position at the foot of the highlands, 【a source of jade, obsidian,
            and other valuable commodities】, to become a commercial power throughout the lowlands

used its position to become, 中间是插入成分，逗号隔开

个人理解

哦,豁然开朗....