cross-sectional study is a kind of study which investigate variables through analyzed the data collected in one specific time for a population. In this study, sample of participants are the same in all variables except one.
in the mentioned research, changing variable is age.
A mutualistic relationship is when two organisms of different species work together an example is a crocodile and plover bird the plover bird eats the pastries out of the crocs teeth and the crocs gets his teeth clean
