Index fossils are used to determine the relative ages of rock and fossils and are also used to define the boundaries between geologic periods.
<u>Option: A</u>
<u>Explanation:</u>
The fossils which are recognized as fossils guides or indicator fossils are used to classify and recognize geological or faunal periods, termed as index fossils. It must be of short vertical reach, wide geographic distribution and swift patterns in evolution. It helps to assess the rock layers ' age and helps to date other fossils found close and around them. For an instance, Ammonites were abundant in the Mesozoic period between 245 to 65 mya, they have not been found after the Cretaceous era, as they became endangered during the K-T extinction (65 mya).
vector is the answer of this blank
Answer: 
Explanation:
The diffraction angles
when we have a slit divided into
parts are obtained by the following equation:
(1)
Where:
is the width of the slit
is the wavelength of the light
is an integer different from zero.
Now, the second-order diffraction angle is given when
, hence equation (1) becomes:
(2)
Now we have to find the value of
:
(3)
Then:
(4)
(5)
Finally:
(6)
Answer: 
Explanation:
This problem can be solved by the following equation:

Where:
is the change in kinetic energy
is the electric potential difference
is the electric charge
Finding
:


Finally:
