1 . Schools across the US East Coast canceled outdoor activities, commercial airline traffic was slowed and millions of Americans were urged to stay indoors on Wednesday as smoke from Canadian wildfires drifted south, blanketing cities in thick and yellow smoke.

“It’s critical that Americans experiencing dangerous air pollution, especially those with health conditions, listen to local authorities to protect themselves and their families,” US President Joe Biden said.

New York’s world-famous skyline, usually visible for miles, disappeared because of the smoke, which some residents said made them feel unwell.

“It makes breathing difficult,” Mohammed Abass said as he walked down Broadway in Manhattan. “I’ve been scheduled for a road test for driving, for my driving license today, and it was canceled.”

“It looks like Mars out there,” said Fox Forecast Center meteorologist Brian Mastro just before 2 p.m.

New York Goveror Kathy Hochul called the situation an “emergency crisis,” noting that the air pollution index for parts of her state were eight times above normal.

In Bethesda, Maryland, a high school moved its graduation ceremony indoors, while a Brooklyn, New York, elementary school deferred its “Spring Fling” dance party. A school in Montelair, New Jersey, called off a fifth-grade trip to an amusement park.

The smoke was wafting (飘荡) over the US northern border from Canada, where hundreds of forest fires have burnt 9. 4 million acres and forced 120,000 people from their homes in an unusually early and intense start to Canada’s wildfire season.

Tyrone Sylvester, 66, playing chess outside in Manhattan’s Union Square us he has on most days for 30 years, but wearing a mask, said he had never seen the city’s air quality so bad. “When the sun looks like that,” he said, pointing at the smoky sky, “we know something’s wrong. This is what global warming looks like.”

Poor air quality is likely to continue into the weekend, with a developing storm system expected to shift the smoke westward across the Great Lakes and deeper south through the Ohio Valley and into the mid-Atlantic region, AccuWeather said.

On 20th July 1969, Neil Armstrong first set foot on the Moon, after which the Apollo 11 mission landed back to Earth     1     (safe) and successfully. With the development of space travel, people got used to     2     idea of the space shuttle flights and treated them like airplane flights, never     3     (expect) a problem. So, when the Challenger space shuttle took off on 28th January 1986, the world seemed to have lost     4     (it) wonder at the astronauts involved. But it was special because there was a teacher, called Christa McAuliffe,     5     wanted to give two 15-minute lessons from orbit to the students to inspire a new generation of astronauts and space     6     (scientist). Unfortunately, the shuttle exploded just over a minute after taking off in Florida, and all seven astronauts     7     board were killed. The whole world was shocked and it brought home the reality of how complex space flight really was. Space shuttle flights didn’t make any progress for nearly three years until the cause of the disaster     8     (investigate). The space shuttle programme resumed with the successful launch of Discovery on 29th September 1988.

We’ll never forget the     9     (lose) of the Challenger and the seven astronauts. The memory of them will inspire us     10     (join) them in humanity’s greatest journey of exploration and discovery.