Which of the following statements is/are true in relation to the term 'detention period' in a settling tank?

1. It may be determined by introducing a dye in the inlet and timing its appearance at the outlet.

2. Greater the detention period, greater the efficiency of removal of settleable matter.

3. It is the time taken for any unit of water to pass through the settling basin.

4. It is usually more than the flow through period.

Select the correct answer using the codes given below.