[Answer] What percentage of Earth's landmass is tropics?

Step 1 : Introduction to the question "What percentage of Earth's landmass is tropics?"

...Love a good white sand beach? Looking to snorkel to your heart’s content? Then you’re in luck. The tropics make up about 36% of the Earth’s landmass, comprising the Earth’s middle regions between the latitude lines of the Tropic of Capricorn and the Tropic of Cancer. Five of the seven continents feature tropical regions – all except Europe and Antarctica – and around a third of the global population calls the tropics home, which is no surprise, as it offers warm year-round temperatures due to sun exposure and has only two seasons: wet and dry. So, depending on what season you visit the tropics, pack accordingly.

[Answer] Where is the lowest point on Earth's surface?

Step 1 : Introduction to the question "Where is the lowest point on Earth's surface?"

...1. Dead Sea 2. Lake Assal 3. Death Valley 4. Danakil Depression

[Answer] 4. This metal is the fifth most abundant element in the earth's crust, and is an essential constituent of leaves, shells, bone, and teeth.

Step 1 : Introduction to the question "4. This metal is the fifth most abundant element in the earth's crust, and is an essential constituent of leaves, shells, bone, and teeth."

...1. iodine 2. calcium 3. phosphorous 4. zinc Calcium is perhaps best known for its numerous compounds. Common chalk is a compound containing calcium.

[Answer] What is the most abundant gas in the Earth's atmosphere?

Step 1 : Introduction to the question "What is the most abundant gas in the Earth's atmosphere?"

...There's oxygen in Earth's air, which is a good thing for human life. But oxygen makes up only 21 percent of the atmosphere. The most abundant gas found in Earth's atmosphere is nitrogen. It makes up 78 percent of the atmosphere. It's a permanent gas, meaning the percentage found in the atmosphere doesn't change on a daily basis. There are trace gasses making up the remainder of the atmosphere, gases such as ozone, helium, nitrous oxide, methane, neo and carbon dioxide. The concentrations of these gasses are always changing. Source: Reference.com