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，請問選A要表達的語意是什麼，
我是選B，因為我認為running out of islands to build (sth) on (them)
這樣想對嗎?謝謝

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