求指导what引导的主语从句

板栗在奋斗

The period when the great painted caves at Lascaux and Altamira were occupied by Upper Paleolithic people has been established by carbon-14 dating, but what is much more difficult to determine are the reason for their decoration,the use to which primitive people put the caves, and the meaning of the magnificently depicted animals.

• Ahas been established by carbon-14 dating, but what is much more difficult to determine are
• Bhas been established by carbon-14 dating, but what is much more difficult to determine is
• Chave been established by carbon-14 dating, but what is much more difficult to determine is
• Dhave been established by carbon-14 dating, but what is much more difficult to determine are
• Eare established by carbon-14 dating, but that which is much more difficult to determine is
  

不太明白，为什么后面有3个对象，应该是复数，而前面be动词仍然选择用is

bejamin1111

hey，在manhattan认为subject phrase都是singular的；关于这里的谈论Ron有更详细的解释，非常不错；
FYI ：http://www.manhattangmat.com/forums/painted-caves-at-lascaux-and-altamira-t7994.html

板栗在奋斗

bejamin1111 发表于 2013-5-21 22:43
hey，在manhattan认为subject phrase都是singular的；关于这里的谈论Ron有更详细的解释，非常不错；
FYI ： ...

well received. many thx.

kiwifoodtown

manhattan里有这么一段定义，摘录出来希望能有帮助：
Sometime the subject of a sentence is an —Ing phrase or even a whole clause. This sort of subject is always singular and requires a singular verb form.