difficult to; hard to; not easy to ～否認(rèn)undeniable;irrefutable/～開(kāi)發(fā)的煤炭資源stranded coal/ ～理解hard to understand;incomprehensible/～忘懷hard to forget; unforgettable/～為繼difficult to maintain; hard to keep up/～置信incredible;unbelievable/～奏效hard to get results