C,that is needed
选择C,ALL 是主句的主语,is a continuous supply of fuel.是主句的谓语部分.
that is needed 是定语从句修饰all,is 是从句里面主语that的谓语.
B 如果改成the thingS needed 也是对的
D的意思上不确切.
另外,修饰all 的定语从句不能用A.如果用A,全句就是：what is needed is a continuous supply of fuel.就不该有all了.那么what is needed就是主语从句,做is a continuous supply of fuel 的主语.