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 concern (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）。