1. At what age do babies learn to crawl? Does it take longer to learn in the winter when babies are often bundled in clothes that restrict their movement? Data were collected at the University of Denver Infant Study Center, where parents and their babies participated in one of a number of experiments between 1988 and 1991. Parents reported the age (in weeks) at which their child was first able to creep or crawl a distance of four feet within one minute. The researchers planned to use simple linear regression to see if the age at which a baby was first able to crawl could be predicted from the average outdoor temperature six months after each baby's birthdate.

The explanatory variable in this study is

A. the average outdoor temperature six months after a baby's birthdate.
B. the age (in weeks) at which a baby was first able to creep or crawl a distance of four feet within one minute.
C. the extent to which parents honestly reported the age at which their baby was first able to crawl and didn't exaggerate in order to make their baby appear gifted.

2. Has the number of home runs hit by major league teams been increasing over time? For the 41 years from 1960 to 2000, the average number of home runs hit per game per team was computed. A simple linear regression was used to study the trend in the average number of home runs over the period 1960 to 2000, by using year to predict average number of home runs. The following results were obtained.

A 99% confidence interval for the slope of the least-squares regression line is
A. 0.011145 ± 0.00685.
B. 0.011145 ± 0.00873.
C. 0.011145 ± 0.00916.