It ---- ten years since they ----to France.

考点是
since的用法：译作“自从…以来”
(1)接时间点：如 I have stayed in that country since 1995.
要求：谓v.用现在完成时,而且须是延续性动词.
(2)接一段时间 + ago：如 They have lived in that house since two years ago.
要求：谓v.用现在完成时,而且须是延续性动词.
(3)接时间状语从句：如 I have loved her since I met her last time./
Lily has stayed at home since she left school.
要求：主句的谓v.用现在完成时,而且须是延续性动词；从句的谓v.用一般过去
时,而且须是终止性动词.
(4)用于句型“It’s / It has been + 一段时间 + since + 时间状语从句”中,译作“自从…以来,已经…(时间)了”.如 It’s / has been three months since he moved to this city.
要求：主句的谓v.用一般现在时或现在完成时；从句的谓v.用一般过去时,而且
须是终止性动词.
(5)用于句型“一段时间 + has passed since + 时间状语从句”中,译作“自从…以来,…
(时间)已经过去了”.如上个例句还可写成：Three months has passed since he moved to this city.
要求：主句的谓v.用现在完成时,而且动词必须是has passed；从句的谓v.用
一般过去时,而且须是终止性动词.
(6)注：当since用于否定句时,主句的谓v.可以是终止性动词.
如 He hasn’t seen the film since last month.
(7)相关词组：“从那时起”：ever since、since then、from then on.
注：ever since作为一个词组,放句尾可单独使用.如I have given up smoking ever since.另外,还可在since后接句子.since前加ever,只是加重语气而已.如 I have given up smoking ever since I caught a bad cough.
(8) since还可引导原因状语从句,译作“由于；因为”,相当于because.如 Since he was lazy,he didn’t finish his homework.
这里是第四个用法,要么用has been 要么用is