【1】prep.
缩写词 abbr.
1.=preparation名词，准备
2.=preparatory形容词，预备的
3.=prepare 动词,作好准备
4.=preposition 介词

【2】a 形容词，adjective

【3】ad
缩写词 abbr.
1.广告
2.ad 副词

【4】X，字母表第24个字母，罗马字第十，数学中的未知数，错误或删除符号。
1 24th letter of the English alphabet
2HMNnumber[countable] the number 10 in the system of roman numerals
3mathematics[uncountable] technicalHM a letter used in mathematics to represent an unknown quantity or value:
if 3x = 6, x = 2
4on school workSE [countable] a mark used on school work to show that a written answer is not correct