Final answer:
Country blues musicians used bottlenecks to change the timbre and pitch of their guitars, a technique known as slide guitar.
Step-by-step explanation:
Country blues musicians altered the timbre and pitch of their guitars using bottlenecks. This technique, often called slide guitar, involves placing a smooth object, such as a glass or metal slide (often a bottleneck), against the strings of the guitar to create a glissando or a continuous sliding sound. The bottle neck allows the player to vary the pitch by moving it up and down the neck of the guitar. This was a common practice in country blues to add expressiveness and a vocal-like quality to the guitar.
While harmonicas were indeed used in folk music, and amplification was later adopted to increase volume in various musical genres, call and response is a musical form rather than a way to modify an instrument. The use of objects like bottlenecks is a specific adaptation to get a louder and more expressive sound from acoustic instruments, particularly in the context of country blues music where other forms of amplification were not available.