[9급 지방직 영어 2014년 18번]

There are ninety-two naturally occurring elements on Earth, plus a further twenty or so that have been created in labs. Not a few of our earthly chemicals are surprisingly little known. Astatine, for instance, is practically unstudied. It has a name and a place on the periodic table, next to Marie Curie's polonium, but almost nothing else. The problem is __________. There just is not much astatine out there. The most elusive element of all, however, appears to be francium, which is so scarce that it is thought that our entire planet may contain, at any given moment, fewer than twenty francium atoms. Altogether only about thirty of the naturally occurring elements are widespread on Earth.