A number of possible explanations for why manhole covers are round include :

1. A round manhole cover cannot fall through its circular opening, whereas a square, rectangular, or oval manhole cover could fall in if it were inserted diagonally in the hole.

2. Circular covers don’t need to be rotated or precisely aligned when placing them on the opening.

3. A round manhole cover is easily moved and rolled.

4. Human beings have a roughly circular cross-section.

5. Round tubes are the strongest shape against the compression of the earth around them, so the cover of the tube would naturally be round as well.

6. It’s easier to dig a circular hole.

Antarctica is surrounded on all sides by water. The Indian, Pacific and Atlantic Oceans surround it on all sides. These three Oceans all meet at Antarctica. The three oceans are collectively called the Southern Ocean.

This is the area located south of 50° latitude.

The troposphere is the lowest layer of the atmosphere and where most clouds are. That means all weather is in the troposphere. Almost all weather occurs in the Troposphere, basically, the lowest layer that goes up to about 10 - 15 kms.

Many types of renewable energy resources such as wind and solar energy are constantly replenished and will never run out.

Plant development is influenced by quality, quantity and duration of light. Plants cannot get too much light, but they can get too much of the heat energy that comes with the light.

Plants requires what quantity of light?

* Most of the garden vegetables require full sun, i.e 6 - 8 hrs of direct sunlight daily.

* Plants that grow in partial sun only need 3 - 4 hrs of direct sunlight daily.

* While plants that thrive in shade need less than 2 hours of sunlight daily.

Given length of the rectangle = 3 cm

Breadth of the rectangle = 4 cm

Then, the diagonal of the rectangle D = 32 + 42 = 25 = 5

Then, it implies side of square = 5 cm

We know that Area of square = S x S = 5 x 5 = 25 sq.cm.

Here the given number series is 6, 3, 3, 4.5, 10

The odd one in the given series is 10

The pattern it follows is

6

6 x 0.5 = 3

3 x  1  = 3

3 x 1.5 = 4.5

4.5 x 2 = 9

Hence, the odd number is 10.

The common greenhouse gas containing only hydrogen and oxygen is H2O, this is water vapor.

The common greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and ozone.

The composition (chemical formula) of those gases are:

Water vapor: H2O
Carbon dioxide: CO2
Methane: CH4
Nitrous oxide: NO
Ozone: O3.

As you see the only one of those that contains only hydrogen and oxygen is H2O, water vapor.