The rise was particularly noticeable between 1999 and 2002, during which time the use of mobile phones tripled.?

The rise was particularly noticeable between 1999 and 2002, during which time (that) the use of mobile phones tripled.
= The rise was particularly noticeable between 1999 and 2002, during the time (that) the use of mobile phones tripled.
= The rise was particularly noticeable between 1999 and 2002 as the use of mobile phones tripled.
(as 是連接詞 = during the time that)

during which time 中的 which 意指 from 1999 to 2002，用during the time 也可以，此一限定 the time 即是前文所指 between 1999 and 2002. 但是寫作好壞的差異，即在這些枝微末節上，原句使用which 要比 the 好

1.) 我想問一下後面的which可以改成the嗎？ ==> 可以
2.) 或者去掉which time ==> 不可以
3.) 後面放tripled是動詞改成過去分詞嗎？因為是補充說明的句子嗎？ ==>不是，tripled 是過去式動詞

during which time=on which=on the date=when
triped=participle verbal adj. qualifying nn phr,(use of mobile phones),but retaining some properties of a verb=a participial adjective, the (tripled) use of mobile phones.

during which time 不可改為 during the time
但可以改為during that time
因為that = 指上句1999-2002這段"時間"。

問題2假如你指寫成
during which time tripled.
文法上是可以的，但假如這樣全句便沒有"主體"，因為句子的主詞會變成"The rise"，而沒有寫出主體是甚麼rise了，主體其實是(the use of mobile phones)。除非全篇文章中上句已經說明The rise是在說The use of mobile phone，則全句更可以簡化為：
The rise was tripled between 1999 and 2002.
但這句便只是說出事實的資料，沒有作者似乎一層層地加重立體感地表達：
1999至2002年間的增幅特別明顯，當其時的流動電話數量增加了三倍。
把"年份"跟"當其時"重複要讀者注意多一次，先說"增幅明顯"描寫出質感，再實在一點地說出"三倍"的數量感。

這就是"寫作"技巧或style手法怎樣把一樣簡單的fact變為說明，再作為"文字的力量"。