A:house
B:horse
— Excuse me, sir. Please don't take photos in the museum. Look at the sign .
— I'm sorry. I didn't notice it just now.
用电热丝烧水,当电压为U时,t秒内可将水烧开,如果电压降为原来的一半,则同样烧开这些水所需的时间应为( )