【单选题】Many firms give away small quantities of their products as( )so that people can try them before buying.
【单选题】对于一均匀的物质,其 H 和 U 的关系为()。
【单选题】Many firms give away small quantities of their products as ______, so that people can try them before buying.
【单选题】Many firms give away small quantities of their productions as _______ so that people can try them before buying.