What type of visual aid is effective in showing comparisons between two or more items?
a. maps
b. illustrations
c. graphs
d. ch21

we need an attachment to see what you are talking about.

even though you have attached neither the chart nor the generalizations, i have located them online. the right answer is d, since they are asking you about a generalization. you can only infer from the chart that between 2008 and 2009 the number of employed workers decreased significantly (from over 115 millions to 107 millions approximately), which signals some type of economic crisis.  

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when you sing louad enough the vibration of you voice is able to shatter glass so yes vibration is your answer

if this mark brainliest i wolud be happy and have a great day you!