【题目】Friendship has to be ________trust, as we all know.
A.found on
B.believed in
C.founded on
D.believed

【答案】C
【解析】考查被动语态及非限制性定语从句。正如我们都知道的，友谊必须建立在信任的基础上。found on (或 upon)以…为根据，以…为基础。友谊是被建立的，所以要用found的过去分词founded 。