Which of the following statements about deep learning are true?

A$.$ The negative side of ReLU is a dead zone, leading to the gradient becomes $0.$

B$.$ The sigmoid function is better than the ReLU function in preventing the gradient disappearance problem.

C$.$ The long short$-$term memory $($LSTM$)$ adds several channels and gates based on the recurrent neural network $($RNN$)$

D$.$ Gated recurrent unit $($GRU$)$ is a simplified version of LSTM$.$