A:is easy to look after
B:are easy to look after them
C:are easy to look after
D:is easy to look after it
____
科学研究证明,烟草燃烧时,烟雾中含有多种对人体有害的物质,如____、____等。这些物质进入人体,不仅能诱发多种____疾病,还有诱发____。