Let T:V→WT: V \to WT:V→W be a linear transformation. If ker(T)={0⃗}\text{ker}(T) = \{\vec{0}\}ker(T)={0} and dim(V)=dim(W)\text{dim}(V) = \text{dim}(W)dim(V)=dim(W), then TTT is:
An isomorphism.
Only one-to-one but not onto.
Only onto but not one-to-one.
Not enough information.