hold up指用暴力或武力威胁抢劫,有明确目的
hold back扣住,保留抑制的意思
1. hold up常表示下列意义：
1) 延迟,阻碍
My application was held up by the postal strike.
2) 搀扶；支撑；举起；抬起
He held up his hand to agree to the plan.
3) 举出（例子）；提出（作为榜样）
She is always holding up her children as models of good behavior.
4) 支持住；承受住；支撑得住
She’s holding up well under the pressure.
2. hold back常表示下列意义：
1) 阻碍,阻止
He was so angry at the news that I couldn’t hold him back.
2) 妨碍进展
Do you think that mixed ability classes hold back the better students?
