Each state of the United States selects two senators to represent their states in the United States Senate. In 1913, the 17th Amendment[?] to the United States Constitution provided for the direct popular election of senators; prior to this, senators were selected by state legislatures.
... was confirmed 20 years later by using the O18 isotope of oxygen as a tracer label to follow the fate of oxygen atoms during photosynthesis.
Light-depend ...