a hard nut to crack

棘手的问题

a, a (large) number of, a bill of fare, a bit, a case in point, a couple of, a far cry, a few, a fifth column, a fortiori, a gentleman's agreement, a good deal, a good few, a good many, a great deal, a great deal of, a great many, a great/good deal of, a kind of, a la carte, a la mode, A level, a little, a lot of, a matter of, a mite, a mountain of, a number of, a point of view, a posteriori, a priori, a problem (for sb.) to do, a scrap of, a series of, a sword of `Damocles, a variety of, a wilderness of, a(an),