An English grammar question is confusing me again. While learning English, I ran into a question from an English learning book, which confuses me a lot. The question says, among the five phrases, there is one that sounds unnatural and incorrect in grammar. However, they all sound perfectly natural to me. So, could you tell me the answer and what should it be changed into? Here is the text: "Westerners, especially Americans, [tend to think] of a friend as someone [whose] company they enjoy. A friend can [ask for] help if necessary, but it [is considered] poor form to cultivate a friendship primarily for what can [be gained] from that person." a. tend to think b. whose c. ask for d. is considered e. be gained