a$.$ In sequential order, the four phases of the business cycle are

expansion, recession, peak, and trough.

trough, peak, expansion, and recession.

recession, trough, peak, and expansion.

peak, recession, trough, and expansion.

b$.$ The length of a complete business cycle

is generally about $3$ years.

varies from about $1$ to $2$ years to as long as $5$ years.

varies from about $2$ to $3$ years to as long as $10$ years.

varies greatly in duration and intensity.

C$.$ Seasonal variations and long$-$run trends compligate the measurement of the business cycle because

long$-$term trends have only been measured recently.

seasons vary so the changes are not consistent.

normal seasonal variations do not signal boom or recession.

it is difficult to treat all the variations the same when the causes are different.