------,I think,and the problem could be settled soon.

选D.比较接近口语
A.So long as you keep spirits
B.If you double your efforts
都是引导条件句的,
因为有了and,所以不可能是从句
Making great efforts 是非谓语,就算没有and,这样的表达也不对.现在有
and,更不可能了.
根据排除法选D
A bit more efforts 这里是祈使句,“只要再多一点努力”
and这里是并列连词,用来连接另一句句子the problem could be settled soon