This is a question with answer options. Goal: How do ensure the shelves will fit?

Which of the following methods is more reasonable for accomplishing this goal?

Choose your answer from:
(I). Use a ruler or measuring tape to mark the locations using a pencil where the rails will be installed.;
(II). Use a shelf to approximate and to mark the locations using a pencil where the rails will be installed.;...I think the answer is
(I).