Fill in the blanks with an appropriate tense form.

The United Kingdom ………..1……… the first country in the world to allow scientists to create human-animal embryos for research purposes. The research ……..2……… transferring nuclei containing DNA from human cells to animal eggs that ………3…….. nearly all their genetic information removed. The resulting embryos ………..4……….., therefore, mostly human, with a small animal component. Stem cells, which ……..5…….. grow into different kinds of tissue, are then ……….6………. from these embryos. The embryos could give researchers a large supply of stem cells to work with. Scientists have, until now, ………..7……….. on human eggs left over from fertility treatment, which are in short supply and often of poor quality.

1. (a) became; (b) has become; (c) have become; (d) become
2. (a) involve; (b) involves; (c) is involving; (d) has involved
3. (a) have; (b) are; (c) have had (d) had
4. (a) is; (b) are; (c) have; (d) have been
5. (a) may; (b) will; (c) can; (d) could
6. (a) harvesting; (b) harvested; (c) harvest; (d) harvests
7. (a) rely; (b) relies; (c) relied; (d) relying

Answers

1. has become, 2. involves, 3. have had, 4. are, 5. can, 6. harvested, 7. relied