— Susan stays alone in the classroom and looks sad.
— _________ to her?
A:What happens
B:What has happened
C:What is happening
D:What was happening
某导体的电阻为40Ω,通过的电流为0.2A,则该导体两端的电压是多少?
函数y=x2+x-1≤x≤3的值域是( )
根据现代生物进化理论判断,下列叙述正确的是
鲜蛋呼吸时会排出二氧化碳,下列物质中能在蛋壳表面形成难溶性固态保护膜,因而可作为鲜蛋保鲜剂的是( )