21.over the period from...to...the...remained level.
在......至......期间,......基本不变。
22.in the year between...and...
在......年到......期间......
23.in the 3 years spanning from 1995 through 1998...
1995年至1998三年里......
24.from then on/from this time onwards...
从那时起......
25.the number of...remained steady/stable from (month/year) to
(month/year).
......月(年)至......月(年)......的数量基本不变。