The companies are working together to create _______ they hope will be the best means of transport in the 21st century.

A.which

B.that

C.what

D.who

答案及解析： C

考查名词性从句。从create一词可判断本句为宾语从句。在从句中they hope为干扰因素，它在句子中可视为插入语，这样就很容易看出从句中没有主语，故B项错。A项which后面一般需要加名词，如which room you live in；D项who指代人，本题易。