One way that unity is often achieved in the classical symphony is by the use of the same

The answer is key

Explanation:

One way that unity is often achieved in the classical symphony is by the use of the same key

key in three of its four movements.

Explanation:

During the Classical and Romantic period, and until today, a symphony is usually played on the same key, along 4 movements.

Sometimes there are variations on the key, especially for the second movement, but traditionally, the four movements form a unity when the key is usually only partially played with a relatively minor or dominant on the second movement, often in a slower tempo.

Since four movements are the common format for symphonies, there are changes in tempo that give a lively continuation of the main theme.

A standard set of tempos as shown below, among the continued use of a key convey a pice the sense of a whole.

Usually, it can be done in this format:

1st movement - allegro (fast) in sonata form

2nd movement - slow

3rd movement - minuet (a dance with three beats in a bar)

4th movement - allegro

Examples:

3) Mozart – Symphony No. 41 (1788)

2) Beethoven – Symphony No. 9 (1824)

1) Beethoven - Symphony No. 3 (1803)

this exemplifies - rondo form