Okay, let's go through these questions:
1. **c. were you doing / started** - "What **were you doing** when it **started** raining?" This uses the past continuous to describe an action in progress when another action interrupted it (simple past).
2. **a. were / having** - "We **were having** lunch when they attacked, we were taking a rest." Similar to question 1, past continuous for an ongoing action interrupted by another action in the past.
3. **d. to themselves** - "The old soldiers of war time sometimes try to hold the ceremonies **to themselves**." This means they keep the ceremonies private or personal.
4. **b. was starting / arrived** - "Just as the fire **was starting** in the dining room, the fire fighters **arrived**." Again, past continuous interrupted by simple past.
5. **c. I am having a hybrid car now.** - This sentence is wrong because "having" in this context (possession) is not usually used in the continuous form. The correct sentence would be "I have a hybrid car now."
6. **c. when** - "I was reading an interesting detective story **when** I heard a terrible noise." "When" indicates the timing of the interruption.
7. **d. were struggling** - "During the world war Il, while many people **were struggling** to keep their families alive, the governments were busy preparing for more battles." Past continuous describes an ongoing situation.
8. **a. were building** - "Nobody could ever imagine how they **were building** such tall buildings at that time while no technologies were available." Past continuous suggests the process of building.
9. **b. was raining** - "I don't know how he managed to control the car while it **was raining** heavily and nothing was clear enough to see." Past continuous to describe the weather condition.
10. **c. believe** - "I am sure, actually I strongly **believe** that future will be ours if only we try our best." "Believe" is the most appropriate word here to express a strong conviction.
11. **d. were collecting / picked** - "Some women and children **were collecting** nuts and other dried fruits from the ground while the men **picked** fruit from the trees." Past continuous for the women and children's ongoing activity, simple past for the men's completed action.
12. **b. was writing / was reading** - "When I arrived in class, Jack **was writing** on the board and Mike **was reading** a comic book." Past continuous describes both actions happening simultaneously.