During which eon did oxygen begin to build up the most in Earth’s atmosphere?
A) Proterozoic
B) Archean
C) Hadean
D) Phanerozoic

the beginning of the iditarod trail sled dog race. the iditarod trail sled dog race first ran to nome in 1973.   the trail they used is today known as the iditarod trail, first surveyed by the alaska road commission in 1908 and now one of the national historic trails as so designated by the congress of the united states.

what inspired

the children of nome were dying in january 1925. infected with diphtheria, they wheezed and gasped for air, and every day brought a new case of the lethal respiratory disease. nome’s lone physician, dr. curtis welch, feared an epidemic that could put the entire village of 1,400 at risk. he ordered a quarantine but knew that only an antitoxin serum could ward off the fast-spreading disease.the nearest batch of the life-saving medicine, however, rested more than 1,000 miles away in anchorage. nome’s ice-choked harbor made sea transport impossible, and open-cockpit airplanes could not fly in alaska’s subzero temperatures. with the nearest train station nearly 700 miles away in nenana, canine power offered nome its best hope for a speedy delivery.sled dogs regularly beat alaska’s snowy trails to deliver mail, and the territory’s governor, scott c. bone, recruited the best drivers and dog teams to stage a round-the-clock relay to transport the serum from nenana to nome. on the night of january 27, 1925, a train whistle pierced nenana’s stillness as it arrived with the precious cargo—a 20-pound package of serum wrapped in protective fur. musher “wild bill” shannon tied the parcel to his sled. as he gave the signal, the paws of shannon’s nine malamutes pounded the snow-packed trail on the first steps of a 674-mile “great race of mercy” through rugged wilderness, across frozen waterways and over treeless tundra.even by alaskan standards, this winter night packed extra bite, with temperatures plummeting to 60 degrees below zero fahrenheit. although every second was precious as the number of confirmed cases in nome mounted, shannon knew he needed to control his speed. if his dogs ran too fast and breathed too deeply in such frigid conditions, they could frost their lungs and die of exposure. although shannon ran next to the sled to raise his own body temperature, he still developed hypothermia and frostbite on the 52-mile leg to tolovana before handing off the serum to the second dog team.

the correct answer is 50.