直接碘量法是利用（）作标准滴定溶液来直接滴定一些（）物质的方法，反应只能（）在（）性或（）性溶液中进行。

The trip to the Mystery Caves will take place on Sunday, 27 July. You can book your tickets in Room 188 between 2:30 p.m. and 4:00 p.m. every day. Your parents should confirm (确认) the booking by writing to us as soon as possible. You should pay for the trip before 21 July. If we do not receive your money early enough, your booking will be cancelled(取消). If the trip does not take place for any reason, your money will be paid back.
We are planning to meet outside the railway station on Black Street at 8:00 a.m. It will take us about 2 hours to get to the center of the town by train, and from there we have to catch a bus. It will take another hour to get to the Mystery Caves on the bus.
We will spend about 1 hour and a half visiting the caves (山洞), and we hope to catch the 1:30 bus to Happy Lake for some swimming. If we are late, we will have to take the 3:00 bus straight back to Black Street.

If you miss the 1:30 bus to Happy Lake, you will
[A] have to go back by train
[B] not be able to go swimming
[C] go to the lake on foot