The noun <em>envelopes </em>is the direct object in this sentence.
I believe the answer is: <span>longitudinal; cross-sectional
Longitudinal study is initiated if the researchers want to collect long term data from the subject as over time (as they grow up).
Cross-sectional study on the other hand is initiated if the researchers want to know whether a certain things only affect a specific population.
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Washington D.C is one of the most important cities in the world because it is the capital of one of the world's largest super power, United States of America.
