The Age of Exploration: Chapter 1, Exercise 2 — Possible Solution
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QUALIFICATION
The source says the Ottomans controlled routes after 1453, but it dates the
start of Portuguese Atlantic exploration to about 1418, before that. It does
not say the Ottoman conquest caused the voyages, and it gives trade with Asia
as the general motive.

REWRITE
"European seafarers sought sea routes to Asia for spices and luxury goods; the
Portuguese began systematic Atlantic exploration about 1418, and the Ottomans
controlled the routes after 1453."

CAUTION
I did not research how historians rank the causes.