连续独立地投两次硬币，令A
1
={第一次出现正面}，A
2
={第二次出现正面}，A
3
={两次中一次正面一次反面}，A
4
={两次都出现正面}，则( )．
【正确答案】
B
【答案解析】解析：P(A
1
)=P(A
2
)=1／2，P(A
3
)=P(A，
=1／2，P(A
4
)=1／4，P(A
1
A
2
)=1／4，P(A
1
A
3
)=P(A
1
)=1／4，P(A
2
A
3
)=P(
=1／2，P(A
4
)=1／4，P(A
1
A
2
)=1／4，P(A
1
A
3
)=P(A
1
)=1／4，P(A
2
A
3
)=P(