Explanation:
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An elements is called a pure substance because it is composed of only one kina of atoms in a fix proportion. 
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It is true that constructive interference will cause two sound waves to cancel each other out resulting in silence. 
        
             
        
        
        
CORRECT ANSWER:
a- Cell-surface receptors bind polar signaling molecules; intracellular receptors bind nonpolar signaling molecules.
STEP-BY-STEP EXPLANATION:
The complete question from book is 
According to Figure 9.6, what is a key difference between cell signaling by a cell-surface receptor and cell signaling by an intracellular receptor?
a- Cell-surface receptors bind polar signaling molecules; intracellular receptors bind nonpolar signaling molecules.
b- Signaling molecules that bind to cell-surface receptors lead to cellular responses restricted to the cytoplasm; signaling molecules that bind to intracellular receptors lead to cellular responses restricted to the nucleus.
c- Cell-surface receptors bind to specific signaling molecules; intracellular receptors bind any signaling molecule.
d- Cell-surface receptors typically bind to signaling molecules that are smaller than those bound by intracellular receptors.
e- None of the other answer options is correct.
 
        
             
        
        
        
Answer:
Explanation:
thickness of oil t = 200 nm 
index of refraction μ = 1.5 
For transmitted light :---
path difference = 2μ t 
For constructive interference 
path difference = n λ , λ is wavelength  of light 
2μ t = n λ 
λ = 2μ t /  n
For longest λ , n = 1 
λ = 2μ t 
= 2 x 1.5 x 200 nm
= 600 nm 
Wavelength in water
= 600 / refractive index of water 
= 600 / 1.33
= 451.1 nm Ans