$4.$ Plot from $1970($as in Figure $7\mathrm{.}7$A$)$ the difference between the $10-$year Treasury yield $($FRED code: GS$10)$ and the $3-$month Treasury bill rate $($FRED code: TB$3$ MS $5.$ Download the data used in Data Exploration Problem $4$ and $($a$)$ find the most recent period for which the yield curve was $($approximately$)$ flat and $($b$)$ the longest time period for which yield curve was inverted. $($LO$3,$ LO$4$