Pick from:
(a). Baking soda should never be applied directly to the grout, as it can cause it to crumble or erode. Water and baking soda make great household cleaners because they keep clean..
(b). It also has a tendency to create harsh odors. Soak or dab a cotton ball with a vinegar solution..
(c). Apply the paste to the grout lines with your finger. Although the baking soda isn't harmful, wearing rubber gloves can help prevent scratches or irritated skin from the abrasive grout and baking soda..
(d). You can also consider removing stones that have deep crevices such as nebulae deposits or quartz, as these can harbor algae. Apply the paste directly to the grout, then let it sit for 15 minutes before scrubbing up in any direction..Choose from options above and answer: What most naturally follows?

How to clean grout between floor tiles
Make a paste of baking soda and water.
Mix 3 parts of baking soda to 1 part water to make a thick paste. This versatile cleaner will work with all colors of grout, but vinegar can be harmful to some natural stones such as marble or limestone.
Answer:
(c).