1.  When it comes to a certain period in life, love relationship will become necessarily.

2.  First, they think when it comes to a certain period in life, love relationship will become necessary and meaningful.

3.  Many countries have such immigration policies as skilled migration which allow foreign students to apply for permanent residency after studying or working full time there for a certain period.

4.  In a certain period, physics should owe its development to the data of astronomical observation.

5.  It is also possible that in a certain period only 20 mopeds are rented per day.

6.  It's universally acknowledged that the productivity of the employees doesn't depend only on the length of the time but also on the output of a certain period.

7.  But I managed to strike a balance between them by being only concentrated on one thing during a certain period.