To prevent contamination of glassware, you should (select all correct answers) only use clean glassware. only pour clean excess chemical back into the reagent bottle. always replace container caps immediately. never pour reagents back into the storage container.

In order to avoid the contamination of the glasswares in the laboratory, the person should always use clean glasswares for the experiments. Using clean glassware every time ensures that no contaminants are being introduced in the experiment that might result in skewed results.

The pouring of the chemicals in the reagent bottle will most likely introduce contaminants into the reagent bottle. So, it should not be done.

The replacement of the container caps will avoid the contamination of the reagents but will not avoid the contamination of the glassware.

Hence, the answer is 'only use clean glassware'.