Medical researchers are painfully aware that there are many problems _____(他们至今都没有找到答案)
一定是which thay haven't found answers to
能不能是and they haven't found answers yet.
这句的意思是这些 many problems 是他们至今都没有找到答案的 problems，所以说"他们至今都没有找到答案"是用来修饰 problems的, 那就要用定语从句修饰problems了，所以一定要用which thay haven't found answers to。and是并列结构，起不到修饰作用。
_______ (大多数父母所关心的) is providing the best education for their children
What most parents care about
没有about行不行？
可以，care本身就是及物动词，后面能直接跟宾语。
Throught the project ,many people have received training and ___(决定自己创业)
decided to start their own business.
为什么不是they decided to start their own business.???and之后不就是另一句话了吗？
and 是并列结构，前后不一定都是句子做并列成分的，比如 I have an apple and a pear.前后都是名词结构。 I have read the book and finished my homework. 同样都不是完整的句子结构来做并列成分的。
The anti-virus agent was not known _____(直到一名医生偶然发现了它)
until it was accidentally found by a doctor
能不能是a doctor find it accidentally.
可以，但是find要改为 found，过去式。
until 作为连词，其后所跟的是一句完整的句子，其被动主动都不会被其前后句子所限制。
until 此外也能做介词，后跟名词作宾语。
希望能帮到你。