选B。原因如下：

add 用作及物动词，表示"加；增加",常与介词to连用,即" add ... to ...","把......加到......中去；往......中加......"。例如：

If you add 4 to 6, you get 10. 4加6等于10。

Add some hot water if you think the coffee is much too strong. 如果你觉得这咖啡太浓,就加点儿热水。

add to（add 作不及物动词用）表示"增加；增添",其中to是介词,后接名词或代词。有时也可用来表示"(房屋的)扩建"或"(书的)续写"等。例如：

Each girl present at the party was wearing a flower in the hair, which added to their beauty. 参加晚会的姑娘人人头上都戴有一朵花,使得她们显得愈加漂亮。

The soldiers were extremely tired and the heavy rain added to their difficulty. 战士们累极了,而大雨更是增加了他们的困难。

This old house has been added to from time to time. 这幢老房子曾一再扩建。

Has that book of his been added to later? 他的那本书后来曾续写过吗？