___ both sides accept the agreement will a lasting peace be established in this region.A.unless B.As long as C.If only D.Only if Please explain.

选D
首先这个句子是个倒装句子，will提前了，正常的语序是---both sides accept the agreement a lasting peace will be established in this region.因此可以判断空缺地方应该填得词是那些要句子倒装的词放在句首的时候如，否定词(no,hardly,seldom,unless等），所以我们排除B,C在根据意思选择D 。意思是：要是双方都同意就好了，持久的和平将在这个地区确立下来。

C， 只有两边都同意，才有可能在这个国家建立长久的和平。
后面是个倒装句，will 提前，所以前面应该是虚拟语气。
If only接虚拟。

D only if:
意思是只有当(只是在...的时候)
只有两边都同意,才有可能在这个国家建立长久的和平.
if only:要是...多好
Examples:
1.If only he had been here.
要是他当时在这儿就好了.
2.I dread the coming exams.If only I could have time to review my lessons.
我对即将到来的考试非常担心.如果我有时间复习该有多好啊.
3.If only I had another chance.
要是我再有一次机会就好了.
4.I am so tired.If only I can take the day off tomorrow.
太疲倦了.如果明天能不上班多好.
5.If only problems would come one at a time!
要是问题一次只发生一个该多好!
6.If only it clears up,we'll go.
只要天一放晴,我们就去.
7.If only one had an unlimited supply of money!
要是财源不断就好了!
8.She would be a very good player if only she could get it together.
她若能控制得当就能成为很好的选手.
only if:
conj.只有当(只是在...的时候)
Examples:
1.The company will succeed only if it have sufficient backing.
只有当该公司获得足够的资助,它才会成功.
2.A compound proposition that has components joined by the word and or its symbol and is true only if both or all the components are true.
综合命题由和或其符号所连接的子句组成的复合命题,只有当所有子句为真时,该句才为真
3.I can buy the house only if a mortgage for 2000 dollars is available.
只有拿到两千美元的抵押贷款,我才买得起那栋房子.
4.Only if I get a job will I have enough money to go to school.
我只有找到了工作才有足够的钱上学.