Questions

Trees that lose all of their leaves for part of the year are known as deciduous trees. Those that don't are called evergreen trees.

Let the missing numbers be x, y, a and b

x + y = 8 ---- (1)
a - b = 6 ---- (2)
x + a = 13 --- (3)
y + b = 8 ---- (4)

2x + a + y = 21 --- From (1 & 3)
a + y = 14 ----  From (2 & 4)

Substitute (2 & 4) in (1 & 3)

2x + 14 = 21
2x = 7
x = 3.5

From (1) ---> 3.5 + y = 8
=> y = 8 - 3.5 = 4.5

a + y = 14 => a = 14 - y = 14 - 4.5 => a = 9.5
From (4) ---- y + b = 8 => b = 8 - y = 8 - 4.5 = 3.5

Hence,
x = 3.5
y = 4.5
a = 9.5
b = 3.5

1848 AD

Mount Kilimanjaro

The digestion process starts in the mouth and the complete digestion process completes in the small intestine where all the nutrients in the food are absorbed.

DNA is not a single stranded and does not have amino acids. Hence options, b & C are eiminated. Dna is a double helix structure containing nucleobase—either cytosine (C), guanine (G), adenine (A), or thymine (T)—as well as a monosaccharide sugar called deoxyribose and a phosphate group. 

Climatology is a study of climate which is subspecialty of environmental science studies.