Atmospheric Levels of Oxygen Gas Approached Modern Levels Approximately ______

question 52

Multiple Choice

Atmospheric levels of oxygen gas approached modern levels approximately ______ years ago.

• A)5 billion
• B)500 million
• C)50 million
• D)5 million
• E)500 thousand

---

Definitions:

Experimental Study

A research method where the researcher manipulates one variable to determine if it causes a change in another variable.

Observational Study

A study where the researcher observes and records data without influencing the outcomes.

Self-Administered Survey

A data collection method where respondents fill out the questionnaire on their own, without the direct assistance of a researcher.

Demographic Variables

Characteristics of populations used in social science research, such as age, gender, income level, marital status, ethnicity, and education level.