用适当的介词或副词完成下列句子。 

1. There was nobody in the empty street ____ whom he could turn for help.

2. Mandela was generous ____ his time, ____ which I was very grateful.

3. We were put in a position ____ which we had either to accept we were less

    important or fight the government.

4. However, in 1963 I helped him blow ____ some government buildings.

5. The address ____ which all letters were sent was in South Africa.