A student is trying to use Dijkstra's algorithm to find the shortest path between A and $F$

They start by making A the current vertex and assigning a tentative distance of $5$ to $B$ and a tentative distance of $6$ to $C.$ Then they mark $A$ as visited, and since $B$ has the lowest tentative distance, they mark B as the current vertex.

Next, they update the tentative distance of $C$ to be $5+2=7,$ and the tentative distance of $D$ to be $5+3=8.$ Then they mark $B$ as visited and select $C($which has the lowest tentative distance$)$ as current.

What mistake did this student make? Answer in a sentence or two