— Children today find it hard to focus on study.

— Exactly, as they ________ so many fun things to do.

选择括号内恰当的单词或词组填入空白处，使句意完整、正确。 
Are you going to____ （join / take part in）the school rowing club?
I think the computer is ____（used / useful）.
I spent two hours____ （sweep / sweeping）the house yesterday.
The house is so expensive that I can't____ （share / afford）it.
Even scientists aren't sure ____（if / whether）or not there are UFOs.

1815年英国议会颁行“谷物法”，规定只有在小麦价格超过每夸特（计量单位）80先令（货币单位）时，才准许进口谷物（现实中小麦价格一般在50～60先令之间）；到1846年英国议会又宣布废除“谷物法”。导致这一变化的主要原因是（     ）