I need to fill the gaps

What is created when solids,liquids,an gases mix with one another

Why is it important always to use horizontal bars in unit fractions when performing unit conversions?

What changes occur in the brain during the teenage years? 25 points, someone me out!a.) thinking becomes slower as the brain grows larger. b.) connections between neurons become stronger and faster. c.) unused neurons fuse together to form larger, better functioning neurons. d.) more neurons are developed.