The file P$12\_68.$xlsx lists monthly and annual values of the average surface air temperature of the earth $($in degrees Celsius$).($Actually$,$ the data are indexes, relative to the period $1951-1980$ where the average temperature was about $14$ degrees Celsius. So if you want the actual temperatures, you can add $14$ to all values.$)$ A look at the time series shows a gradual upward trend, starting with negative values and ending with $($mostly$)$ positive values. This might be used to support the claim of global warming. For this problem, use only the annual averages in column N$.$

a$.$ Is this series a random walk?