Heart Rate and QT Interval The heart rates (in beats per minute) and QT intervals (in milliseconds) for 13 males (the figure at the left shows the OT interval of a heartbeat in an electrocardiogram) (Adapted from Chest)
$$\begin{array}{|l|c|c|c|c|c|c|c|}
\hline \text { Heart rate, } \boldsymbol{x} & 60 & 75 & 62 & 68 & 84 & 97 & 66 \\
\hline \text { QT interval, } \boldsymbol{y} & 403 & 363 & 381 & 367 & 341 & 317 & 401 \\
\hline
\end{array}$$
$$\begin{array}{|l|l|l|l|l|l|l|}
\hline \text { Heart rate, } \boldsymbol{x} & 65 & 86 & 78 & 93 & 75 & 88 \\
\hline \mathbf{Q T} \text { interval, } \boldsymbol{y} & 384 & 342 & 377 & 329 & 377 & 349 \\
\hline
\end{array}$$
(a) $x=120$ beats per minute
(b) $x=67$ beats per minute
(c) $x=90$ beats per minute
(d) $x=83$ beats per minute