1.A number of factors are accountablefor this situation.

　　A number of factors might contributeto the phenomenon .

　　2.The answer to this problem involvesmany factors.

　　3.The phenomenonmainly stems from the fact that...

　　4.The factors that contribute to this situation include...

　　5.The change in ...largely results from the fact that...

　　6.We may blame ...,but the real causes are...　

　　7.Part of the explanationsfor it is that ...

　　One of the most common factors (causes .is that ...

　　Another contributing factor (cause .is ...

　　Perhaps the primaryfactor is that …

　　But the fundamental cause is that