Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Experimental group 1 | Experimental group 2 | Variable that is manipulated in experiment |
| --- | --- | --- |
| plants near a sunny window | plants in a dark room | light |
| plants at high altitude | plants at low altitude | air |
| plants watered weekly | plants watered monthly | water |
| plants in sand | plants in garden soil | soil |
Question:
Which of the following groups can be a control group to test the effect of air on plant growth? Select the correct choice to answer the question or fill in the blank.
Choices:
(A) plants in a dark room
(B) plants at low altitude
(C) plants watered monthly
(D) plants in garden soil
Answer:
(B) plants at low altitude