Some of the world's most luxurious resorts are on islands. Tropical
beaches and sparkling waters make up a popular getaway for so many
people around the world that construction companies are running
out of islands to ___.
(A) build up (B) build on
答案是A，请问选A要表达的语意是什么，
我是选B，因为我认为running out of islands to build (sth) on (them)
这样想对吗?谢谢

build up: Establish and develop (a business oror situation) over a period of time