Which of the following is$/$are true about PCR$?$ Choose all options that are true.

b$.$ PCR reduces the number of original features used $($p$)$ because we know that M $<$ p

c$.$ The number of principal components M can be chosen using cross$-$validation

a$.$ The principal components are orthogonal to each other

d$.$ The first principal component is the eigenvector of the covariance matrix associated with the smallest eigenvalue