Please fetch the bunch of purple flowers and put it here, for purple flowers look more beautiful _____ a light yellow wall.
| A．at | B．in | C．under | D．against |
It is believed that forest bathing ______ strengthen the immune system.
| A．should | B．must | C．need | D．can |
As long as the laws of nature ______, there will be tornadoes.
| A．exist | B．are existing | C．will exist | D．existed |
North Korea fired a long-range rocket last month, but the launch was reported ______ in failure.
| A．to end | B．to have ended | C．having ended | D．ending |
Some scientists have questioned the theory ______ CO2 is causing dangerous global warming.
| A．which | B．what | C．that | D．why |
They studied what ______ since 2007 and found Australian electricity prices were among the highest in the developed world.
| A．has happened | B．happened | C．had happened | D．has been happening |