Don't be afraid of asking for help __________ it is needed．

答案D
该题考查从属连词的用法.unless引导一个否定的条件状语从句；since引导时间状语从句表示“自从……以来”,引导原因状语从句表示“因为,既然”；although引导让步状语从句表示“虽然”、“尽管”.A、B、C项填入空白均与语境不符.此空应填入when表示一个肯定的条件,这时when = if,as long as,意为“如果”、“只要”.答案为D.例如：We'll call on you when you stay at home．
如果你在家我们就来拜访你.
Turn off the switch when anything goes wrong with the machine．
如果机器发生故障,就把电门关上.