yòu
他拿着这封信看了又看
Tā názhe zhè fēng xìn kàn le yòu kàn.
He held this letter and read it again and again.
生物的进化，是一步又一步地由低级向高级发展。
Shēngwù de jìnhuà, shì yí bù yòu yí bù de yóu dījí xiàng gāojí fāzhǎn.
The evolution of organisms is a step-by-step development from lower to higher levels.
又好又快
yòu hǎo yòu kuài
both good and fast