Can pick more answers then one: Which statements are correct about the ReLU activation function?

Question $8$Answer

a$.$

Can be expressed as f$($x$)=$ max$(0,$ x$).$

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b$.$

Is a non$-$linear activation function.

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c$.$

The derivative is undefined at x$=0.$

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d$.$

The derivative is simpler to compute than for a sigmoid function.

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e$.$

Is a linear function.

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f$.$

The function is too inefficient to be used in practice.