Inverse Functions

Which of the following functions have an inverse?
i. 𝑓(𝑥) = 3𝑥 − 1
ii. 𝑓(𝑥) = −5
iii. 𝑓(𝑥) = 2𝑥(𝑥2 − 1)
iv. 𝑓(𝑥) = −𝑥(𝑥 − 4)
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