All tanks shall be given a visual external inspection, by an Authorized Inspector, at what frequency?

Inspection conducted at least every 20 years or at the quarter corrosion rate of the bottom whichever is less

Inspection conducted at least every 5 years or at the quarter corrosion rate of the bottom whichever is less

Inspection conducted at least every 5 years or at the quarter corrosion rate of the shell whichever is less

Inspection conducted at least every 20 years or at the quarter corrosion rate of the shell whichever is less