1.C
此题考查状语从句的省略：如果状语从句的主语是it或与主句主语相同,且从句的谓语动词以be动词开始,则可以把状语从句的主语和be动词省略.
即：Once begun = once it is begun.
2.B
固定句型“It remains to be seen + whether引导的从句”,意思是“...是否...还不得而知”.
再如：It remains to be seen whether you are right.你是否正确目前尚不得而知.
3.C
此题考点同第1题,即使用了状语从句的省略语法.
即：When compared with the size of the whole earth
= When the biggest ocean is compared with the size of the whole earth
另：如果去掉when,直接使用过去分词做状语,也是可以的.
4.B
固定搭配：have sb to do sth,意思是“有某人/物去做某事”或“有做某事的某人”.
再如：I have many workers to work at this project.
我有很多工人来做这项工程./ 我有很多做这项工程的工人.
此结构中的have意思是“有”,并非使役动词“使/让”之意.