40% of the world's land surface are coverd by forests nad grassland 应该是are 还是is

40% of the world's land surface ___ coverd by forests nad grassland .【答案】填is【解析】land surface "陆地表面",是不可数名词“……percent of the +名词”结构若是不可数名词,后面谓语动词为is;若是可数名...