After many failures the most difficult ______ is how to keep our strength up and fight again.

A．chance

B．change

C．challenge

D．challenger

下图是X、Y两种固体物质的溶解度曲线。下列说法中正确的是

A．t1℃时X的溶解度大于Y的溶解度

B．t2℃时X的溶解度等于Y的溶解度

C．t3℃时X的饱和溶液降温至t2℃时，变为不饱和溶液

D．t1℃时Y的不饱和溶液升温至t2℃时，变为饱和溶液