■valuable: a small object, especially jewellery, which might be sold for a lot of money(贵重物品). eg.
recover stolen valuables
语法：总是用复数形式。
■variable: a number, amount or situation which can change(可变的事物; 可变的量). eg.
The variables in the equation are X, Y and Z.
The data was analysed according to neighbourhoods, but other key variables like credit rating, job history, savings and marital status were ignored altogether.
语法：可数，常用复数。
■untouchable: in some societies, someone who belongs to the lowest social class(贱民，不可接触者). eg.
He was born an untouchable in a very poor village in south
语法：可数。
■unmentionable：unmentionable people or things (esp underwear)（不宜提到的人或事物， （尤指）内衣）.
语法：总是用复数形式。