87. Medical researchers are painfully aware that there are many problems (that/which) they haven't found solution to (他们至今还没有答案)。

　　88. What most parents concersn (大多数父母所关心的) is proving the best education possible for their children.

　　89. You'd better take a sweater with you in case it (should) turn cold.(以防天气变冷)

　　90. Through the project, many people have received training and decided to launch their own enterprose (决定自己创业)。

　　91. the anti-virus agent was not known until a doctor discovered it by chance.(直到一名医生偶然发现了它)。