I can still remember that it is since the 1990’s, _____ we began to use cell phones,_______ the number of mobile users has greatly increased．

       A．while; when    B．when; what     C．before; that     D．when; that

I can still remember that it is since the 1990’s, _____ we began to use cell phones,_______ the number of mobile users has greatly increased．

I can still remember that it is since the 1990’s, _____ we began to use cell phones,_______ the number of mobile users has greatly increased．

1. A.
   while; when
2. B.
   when; what
3. C.
   before; that
4. D.
   when; that

I can still remember that it is since the 1990’s, _____ we began to use cell phones,_______ the number of mobile users has greatly increased．

       A．while; when    B．when; what     C．before; that     D．when; that

I can still remember that it is since the 1990’s, _____ we began to use cell phones,_______ the number of mobile users has greatly increased．