A random sample of 20 customers for a waste disposal company returned the following results for weekly recycling. Both paper and plastic are given in pounds for each household while household is a measurement of how many individuals live in the home full time.

Household Size	Plastic	Paper
2	1.09	2.41
3	1.04	7.57
3	2.57	9.55
6	3.02	8.82
4	1.5	8.72
2	2.1	6.96
1	1.93	6.83
5	3.57	11.42
6	2.32	16.08
4	1.89	6.38
4	3.26	13.05
7	3.99	11.36
3	2.04	15.09
5	0.99	2.8
6	2.96	6.44
2	1.5	5.86
4	2.43	11.08
4	2.97	12.43
3	1.42	6.05
3	3.6	13.61

In the relationship between paper recycled and household size the independent variable is:

a. Household size

b. Paper recycled

c. Plastic recycled

d. Observation number