Which of the following statements is true about the periodic table.
1. It is outdated and most scientists do not use it anymore.
2. Scientists can use it to predict how elements will behave during a chemical reaction.
3. Elements can belong to several different groups at one time.

In this question, the statement 2 is correct and true while other statements are incorrect.

Explanation:

The properties of an element can be easily predicted from the periodic table. We can find out the atomic number of the element from the periodic table. According to the periodic table, the properties of element is dependent on the atomic number of the element. The behavior of element during a chemical reaction can be predicted from the atomic number.

Element can only belong to one group at a time. Hence, statement (3) is not correct.

The periodic table is still used by scientists for predicting the properties. Hence, it is not outdated. Statement (1) is not correct.

Statement (2) is correct

your answer would be a. 12.5 kg